Resizeable Bar Support on the Raspberry Pi(jeffgeerling.com)
jeffgeerling.com
Resizeable Bar Support on the Raspberry Pi
https://www.jeffgeerling.com/blog/2025/resizeable-bar-support-on-raspberry-pi
48 comments
I'm always fascinated by hobbyist's storage compartments. Some of the labels of the boxes in the background:
I think of this as a Midwestern thing, where land is cheap and one can afford to devote a lot of space to hobbies. It seems to be the same thing in fabric arts, where people will have sewing rooms larger than the footprint of many UK houses. I could be totally be off base here. Maybe some actual Midwesterners can weigh in.
Small-town midwesterner here.
There's some truth to what you're saying. Even the smallest apartment I've ever had was big enough to dedicate space to some serious hobbies, and it was (at that time) very inexpensive to rent compared a much smaller place in a more-dense area.
My lifestyle isn't very compact because it has never needed to be compact. Compactness isn't a common expectation 'round these parts. I have space to keep collections of stuff that I find interesting, and to make use of it.
There's tradeoffs to this lifestyle, though. The corner store is only a short walk away, but it mostly just sells beer, soda, and smokes. There's no walking to get something like groceries, or a new shirt (or a used shirt, for that matter). There isn't much for local entertainment. It's 15 miles to the next-largest city, and there's zero public transportation aside from the buses that get kids to/from school.
(It's hard to imagine that there aren't areas of the UK that are of similar form, though, with roomy housing, space for things, and with very limited services and/or options for commerce nearby.)
There's some truth to what you're saying. Even the smallest apartment I've ever had was big enough to dedicate space to some serious hobbies, and it was (at that time) very inexpensive to rent compared a much smaller place in a more-dense area.
My lifestyle isn't very compact because it has never needed to be compact. Compactness isn't a common expectation 'round these parts. I have space to keep collections of stuff that I find interesting, and to make use of it.
There's tradeoffs to this lifestyle, though. The corner store is only a short walk away, but it mostly just sells beer, soda, and smokes. There's no walking to get something like groceries, or a new shirt (or a used shirt, for that matter). There isn't much for local entertainment. It's 15 miles to the next-largest city, and there's zero public transportation aside from the buses that get kids to/from school.
(It's hard to imagine that there aren't areas of the UK that are of similar form, though, with roomy housing, space for things, and with very limited services and/or options for commerce nearby.)
There are certainly places with very limited services! As for roomy houses, though, that is much harder to find. I did a little searching and found a UK govt report on housing sizes [1]. For a point of comparison, average house size broken out by house type is smaller for all types than the average across all house types for Missouri [2; taken as a random exemplar of the Midwest]. Only when we break out to detached houses built in particular time periods (p.18) does the average become roughly equal to Missouri, and remember this is across all housing in Missouri (IIUC).
The UK settlement pattern is quite weird. About 1/6 of the population lives within the South-East, which is roughly the area commutable to London by train. Then another 1/6 of the population lives in London. So land is very expensive there. The rest of the UK is on average a lot poorer. So the UK in general is not conducive to large housing. You can certainly find old farm houses out in the sticks, and small villages that are mostly expensive large houses. These are a tiny fraction of the housing, though. I believe UK houses are on average the smallest in Europe. (I think the UK would benefit a lot from some decentralisation, but that's a different topic. Side eye in cancelled HS2 lines.)
[1]: https://assets.publishing.service.gov.uk/government/uploads/...
[2]: https://fred.stlouisfed.org/series/MEDSQUFEEMO
The UK settlement pattern is quite weird. About 1/6 of the population lives within the South-East, which is roughly the area commutable to London by train. Then another 1/6 of the population lives in London. So land is very expensive there. The rest of the UK is on average a lot poorer. So the UK in general is not conducive to large housing. You can certainly find old farm houses out in the sticks, and small villages that are mostly expensive large houses. These are a tiny fraction of the housing, though. I believe UK houses are on average the smallest in Europe. (I think the UK would benefit a lot from some decentralisation, but that's a different topic. Side eye in cancelled HS2 lines.)
[1]: https://assets.publishing.service.gov.uk/government/uploads/...
[2]: https://fred.stlouisfed.org/series/MEDSQUFEEMO
To be fair, you can also live small town, Midwest and be a block from a grocery store if that is something that you want to choose.
In fact, I am walking to the grocery store as I dictate this to my phone
In fact, I am walking to the grocery store as I dictate this to my phone
There is indeed one grocery store remaining here. This particular town is lucky in that way: Over the past couple of decades it has gone from 3 or 4 grocery stores to 1, instead of from 1 or 2 to 0 like many surrounding communities.
In terms of choice: No, not really. The housing market is very tight these days. So for now, at least: While I must live somewhere, I don't necessarily get to live where I want. I must instead make do with living where I can (and I suppose optionally also maintain hope for a better day tomorrow).
In terms of choice: No, not really. The housing market is very tight these days. So for now, at least: While I must live somewhere, I don't necessarily get to live where I want. I must instead make do with living where I can (and I suppose optionally also maintain hope for a better day tomorrow).
As an EU resident with a modestly large flat who's dedicated 1/3 of it to a large living room because we prioritized room to relax and spend time together, I pine for the chance to have more than a pair of IKEA KALLAX to store (and pile upon) all my electronics stuff and 3D printer(s).
You know, I realized the same thing after seeing a guy in one of our 3D print communities consistently printing life-sized models scaled with LuBan for his students. He had a garage full of Bambus, a bunch of photos of his models in an apparently high-trust community elementary school, and it just clicked when he said "rural Kansas".
We operate one of the largest print farms in the nation, and I can count the number of human-scale or larger sculptures we've put out in the past decade on one hand.
We operate one of the largest print farms in the nation, and I can count the number of human-scale or larger sculptures we've put out in the past decade on one hand.
It's all true, when I moved to the west coast from the midwest I was struck by the way everyone just piled their possessions into a heap in the middle of their workspace rather than placing them in labeled containers. At times the heaps reach the ceiling.
Not just the Midwest, but most of the US outside the East Coast cities tend to have pretty large homes and apartments compared to most of the rest of the world in terms of home sizes. The US is pretty huge, with much of it livable and relatively less total density than other nations.
Of course, the US has also done a lot more than many places to preserve natural wilderness with national and ngo parks/reserves.
My house in Phoenix, AZ, USA is relatively typical 3br/3ba and just over 2K sqft (around 192 M^2), and I know a lot of people with larger homes than this.
Of course, the US has also done a lot more than many places to preserve natural wilderness with national and ngo parks/reserves.
My house in Phoenix, AZ, USA is relatively typical 3br/3ba and just over 2K sqft (around 192 M^2), and I know a lot of people with larger homes than this.
Yeah, I've seen some youtube recommendations pop up for videos about "tiny homes" that would be fairly standard sized homes here.
I mean, yes, housing and land is cheaper in the midwest US, but you mostly have to get at least an hour away from a major city before it becomes what I consider "cheap." (Otherwise you're still looking at more desirable suburban or bedroom community housing prices.)
But to defend Jeff, the pictures in the article show like a 4x4 corner of a room, even if we extrapolate to say, 8x8x8, that amount of space can usually be found in even a 1-bedroom apartment. I have seen many pictures of amazingly complete electronics/hacking workbenches from around the world that easily fit into much smaller spaces.
But to defend Jeff, the pictures in the article show like a 4x4 corner of a room, even if we extrapolate to say, 8x8x8, that amount of space can usually be found in even a 1-bedroom apartment. I have seen many pictures of amazingly complete electronics/hacking workbenches from around the world that easily fit into much smaller spaces.
Jeff is a bit more than a hobbyist, between him & his father, they are rather accomplished professional radio + electric engineers.
Jeff is a hobbyist in that he has no formal technical training or professional experience with electronics engineering (unless you count youtube shenanigans as such).
Not much of a hobbyist in the sense that he earns (part of) his income from dabbling with RPis. And for the rest of his income, uses them to simulate larger clusters, which he writes books and consults about.
Generally, hobbies are activities where one sees loss not profit.
Generally, hobbies are activities where one sees loss not profit.
That's "perf boards" as in perforated. Single-use, solderable prototyping board.
> "Pepp(?) boards"
"Perfboards", perhaps?
"Perfboards", perhaps?
...and they have time and access to label maker, as opposed to sharpie. That's some dedication to having order.
Label makers aren't all that expensive, even for relatively high volume and fancy options. It's been relatively useful IMO to have bins and labels available. That said, my own office space is a bit cluttered right now with a handful of half-completed projects that I've been too lazy to finish.
A label maker is about $20. I’m not sure that speaks to intense dedication all by itself.
I wonder if they have a drawer labeled "Label Makers".
The first thing I have always done with any label maker I have touched is printed a test label saying "label maker" and stuck it to the gadget.
I do, and the back of the P-Touch also has a label, "P-Touch" :)
I painstakingly type out MAC addresses etc. on a retro embossing-style label maker beacuse I prefer the aesthetic. I think that's the point to start making personality judgements!
> That also led me to research why a card with only 12GB of VRAM is requesting a 16GB BAR, and as far as I can tell, it has something to do with potential uses like SR-IOV where multiple VMs could access a single device, thus requiring more BAR space for the physical device being shared... BAR is still a little bit of a mystery to me.
Lengths of BARs have to be powers of two.
Lengths of BARs have to be powers of two.
I am still waiting for Pi5 to correctly support USBC power delivery spec. 5.1 Volts at 5 amps is a dirty hack and fire hazard, not consumer electronics!
Huh? There are plenty of good reasons to complain about the 5V5A thing, but "fire hazard" is not one of them.
It is even entirely within spec for a PD power supply to offer a 5V5A PDO, as long as it is only doing so with a 5A capable cable (i.e. 100W or 240W). 5V5A is no more a fire hazard than 20V5A.
The spec violation isn't that it negotiates 5V5A when available, but that it isn't willing to buck from 9V or 15V to get those 25W which means that power supply compatibility is incredibly limited.
It is even entirely within spec for a PD power supply to offer a 5V5A PDO, as long as it is only doing so with a 5A capable cable (i.e. 100W or 240W). 5V5A is no more a fire hazard than 20V5A.
The spec violation isn't that it negotiates 5V5A when available, but that it isn't willing to buck from 9V or 15V to get those 25W which means that power supply compatibility is incredibly limited.
Shame a 5v "fake" PPS voltage couldn't somehow be obtained or patched in. Loads of chargers would work then.
My pocket PD can request 5v5a from quite a few chargers in PPS mode.
My pocket PD can request 5v5a from quite a few chargers in PPS mode.
It's a shame that RPi didn't just adopt a proper PD interface for power. For that matter, if they had USB-C + TB/USB4 with display support, then I could just plug it into my display without any other cables like I do my laptop, with all the peripherals connected to the display.
Any currently existing (to say nothing of two years ago) "TB/USB4" chipset would dramatically increase the price of something with a retail price on the order of $50.
With that said, DisplayPort Alternate Mode would be considerably more straightforward.
With that said, DisplayPort Alternate Mode would be considerably more straightforward.
Apparently the RPi 5's SoC already supports USB-C display alt-mode.. unfortunately they don't to proper PD negotiation, which would not be considerably more expensive. There are cheap vape pens that support PD negotiation properly.
Are you sure these "cheap vape pens" don't just use 5V3A, which doesn't require any PD negotiation at all? (a lot of them screw even that much up, and a lot of people confuse "PD negotiation" with simply having the right resistors on the CC pins)
There is real cost savings here -- the RPi5 avoided the need for a buck circuit, and for that matter probably a dedicated PD controller chip.
In contrast, in the context of a "cheap vape pen" you have a battery which means you need to be able to convert to (and from!) battery voltage, so you need that conversion circuitry anyway.
There is real cost savings here -- the RPi5 avoided the need for a buck circuit, and for that matter probably a dedicated PD controller chip.
In contrast, in the context of a "cheap vape pen" you have a battery which means you need to be able to convert to (and from!) battery voltage, so you need that conversion circuitry anyway.
That would absolutely be a better solution but I meant in hindsight.
> negotiates 5V5A
Even the voltage is not matching spec (Pi power supply has 5.1 volts, not 5.0 volts!). That is because historically Pi had shitty cables, with high resistance and voltage drops. 5V5A is not even in spec, limit for 5 volts is 3 amps!
> fire hazard than 20V5A
That would be 100 watts! Many people just grab any usbc cable, and solder it directly to GPIO power pins. But good luck with that!
Initial batches of Pi4 did not even had a resistor, to request 3.0 amps!
Even the voltage is not matching spec (Pi power supply has 5.1 volts, not 5.0 volts!). That is because historically Pi had shitty cables, with high resistance and voltage drops. 5V5A is not even in spec, limit for 5 volts is 3 amps!
> fire hazard than 20V5A
That would be 100 watts! Many people just grab any usbc cable, and solder it directly to GPIO power pins. But good luck with that!
Initial batches of Pi4 did not even had a resistor, to request 3.0 amps!
> That would be 100 watts!
The point is that power dissipation in a cable is a function of the current going through it. The cable will get exactly as hot carrying 5 amps with a voltage of 5 volts as it will carrying 20 or 48.
(now, that is more *wasteful* -- you lose the same amount of power to heat carrying 25W at 5V5A as you do at 100W 20V5A, but that's 4x the relative waste in power)
> Many people just grab any usbc cable, and solder it directly to GPIO power pins.
You're not going to get *any* 5 amp mode out of a standard PD power supply unless the cable indicates it is 5 amp capable, which isn't going to happen unless that "any usbc cable" has the right emarker on it.
> limit for 5 volts is 3 amps.
There is no such limit.
What there is is two things: 1. There are a standard set of PDOs a standard "X watt" PD power supply is supposed to provide. 5V3A 9V3A 15V3A 20V5A, (then 28, 36, and 48 volts for EPR) with the highest one limited to the power limit of the supply. These only go up to 3 amps until you get to 20 volts. 2. Devices are supposed to support those standard PDOs.
Anything other than those standard PDOs is optional (at least before 3.2 which starts introducing AVS as a requirement at 27W+). 12V support is common, as for that matter is PPS support. 5A support below 20V in fixed PDOs is 100% allowed but is super rare.
(5A lower voltage PPS is a different story, but unfortunately the RPi5 doesn't know how to negotiate 5V PPS. That is a shame because it would 100x its power supply compatibility because most chargers targeting higher end Samsung phones support it.)
A power supply is 100% allowed to support 5V5A. It just isn't required to. It would have been 100% legitimate for the RPi5 to have a buck circuit to handle a standard 27W 9V3A power supply and then turn that buck off if the power supply and cable support 5V5A.
> Initial batches of Pi4 did not even had a resistor, to request 3.0 amps!
To be precise, it had *a* resistor (connected to the shorted together CC pins) when it was supposed to have one separate resistor for each pin, and that broke cables with emarkers.
The point is that power dissipation in a cable is a function of the current going through it. The cable will get exactly as hot carrying 5 amps with a voltage of 5 volts as it will carrying 20 or 48.
(now, that is more *wasteful* -- you lose the same amount of power to heat carrying 25W at 5V5A as you do at 100W 20V5A, but that's 4x the relative waste in power)
> Many people just grab any usbc cable, and solder it directly to GPIO power pins.
You're not going to get *any* 5 amp mode out of a standard PD power supply unless the cable indicates it is 5 amp capable, which isn't going to happen unless that "any usbc cable" has the right emarker on it.
> limit for 5 volts is 3 amps.
There is no such limit.
What there is is two things: 1. There are a standard set of PDOs a standard "X watt" PD power supply is supposed to provide. 5V3A 9V3A 15V3A 20V5A, (then 28, 36, and 48 volts for EPR) with the highest one limited to the power limit of the supply. These only go up to 3 amps until you get to 20 volts. 2. Devices are supposed to support those standard PDOs.
Anything other than those standard PDOs is optional (at least before 3.2 which starts introducing AVS as a requirement at 27W+). 12V support is common, as for that matter is PPS support. 5A support below 20V in fixed PDOs is 100% allowed but is super rare.
(5A lower voltage PPS is a different story, but unfortunately the RPi5 doesn't know how to negotiate 5V PPS. That is a shame because it would 100x its power supply compatibility because most chargers targeting higher end Samsung phones support it.)
A power supply is 100% allowed to support 5V5A. It just isn't required to. It would have been 100% legitimate for the RPi5 to have a buck circuit to handle a standard 27W 9V3A power supply and then turn that buck off if the power supply and cable support 5V5A.
> Initial batches of Pi4 did not even had a resistor, to request 3.0 amps!
To be precise, it had *a* resistor (connected to the shorted together CC pins) when it was supposed to have one separate resistor for each pin, and that broke cables with emarkers.
Keep waiting. I really hoped they'd not repeat this problem from the 4 to the 5, but they did.
Oh I'm with you. I've been trying to clean up my setup and the Pi5 is still a problem. Even my Intel N100 NUC is using a USB-C to barrel jack adapter and working great with a perfectly normal multi-port GAN charger, but the Pi5? "Undervoltage Detected"
sigh
sigh
BAR mean Base Address Register. The acronym is used constantly in the article but not defined anywhere except for the external links and someone's comment at the bottom.
> Resizeable Bar Support on the Raspberry Pi
Why'd the capitalisation of BAR get lost between the article and here? It's a little triggering.
Other than that, nice to see, a lot of ARM SoCs & SBCs have asininely small PCIe memory map sizes. Looking at you, RK3399, what was it, 64MB? Not even enough for some NICs.
Why'd the capitalisation of BAR get lost between the article and here? It's a little triggering.
Other than that, nice to see, a lot of ARM SoCs & SBCs have asininely small PCIe memory map sizes. Looking at you, RK3399, what was it, 64MB? Not even enough for some NICs.
That's an hn title munging things I think.
Isn't it the DMA engine that's responsible for the data movement? How does the size of the BAR even factor into that?
If you can map all VRAM into host linear/physical address space, you can just use write combining page table entries to it and get efficient buffered write back caching for the CPU writes that created the very bytes you think of DMA-ing in the first place.
Write combined on cpu will batch a few bytes here and there but you’ll never hit full 4k transactions without dma. I brought up a few new gpus on pcie cards and spent many moons staring at interposer dumps.
It’s ok for small things but even once you get into the command buffer range it’s slow slow slow without dma.
It’s ok for small things but even once you get into the command buffer range it’s slow slow slow without dma.
Hold up, when did htop get the ability to graph things?