Re: [PATCH 2/3] ARM: at91: mmc-xload: allow overriding card capacity

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello Alex,

On 03.06.21 08:53, Alexander Dahl wrote:
> thanks for the surprisingly detailed explanation.

:-)

> Am Thu, Jun 03, 2021 at 08:28:48AM +0200 schrieb Ahmad Fatoum:
>> On 03.06.21 07:34, Alexander Dahl wrote:
>>> Am Wed, Jun 02, 2021 at 12:25:23PM +0200 schrieb Ahmad Fatoum:
>>>> The PBL MMC driver works with the assumption that the BootROM has left
>>>> the SD-Card in transfer mode. There seems to be no definitive way
>>>> to find out whether a running card is high capacity (> 2G) or not,
>>>> but we need this info when reading, because default capacities accept
>>>> their read offset in bytes while high capacity deal in 512 byte blocks.
>>>
>>> I'm a little surprised there's not.  Once like ten years ago I had to
>>> write a SD card driver for a small microcontroller.  In the firmware I
>>> switched to high capacity mode just based on the Card Capacity Status
>>> (CCS) bit in the Operation Conditions Register (OCR) of the SD card.
>>> Got that after sending advanced command 41 (send op cond) to the card.
>>
>> barebox proper does that in sd_send_op_cond() as well.
>>
>>> Not sure if it's that easy, or if that command was only sent under
>>> certain conditions, but I can not remember just guessing high capacity
>>> based on some heuristics nor hard code it. 
>>
>> When you build at91_multi_defconfig, multiple images are generated, currently:
>>
>> barebox-at91sam9x5ek.img
>> barebox-at91sam9263ek.img
>> barebox-microchip-ksz9477-evb.img
>> barebox-microchip-ksz9477-evb-xload-mmc.img
>> barebox-sama5d27-som1-ek.img
>> barebox-sama5d27-som1-ek-xload-mmc.img
>> barebox-groboards-sama5d27-giantboard.img
>> barebox-groboards-sama5d27-giantboard-xload-mmc.img
>> barebox-skov-arm9cpu.img
>>
>> This patch here is for the xload-mmc.img's, which result from the barebox
>> prebootloader. The PBL sets up SDRAM and chainloads barebox proper from the
>> same boot medium that itself came from. Because of this limited scope, it can
>> reuse here the SD-card setup done by the BootROM. The BootROM leaves the
>> SD-Card in transfer mode, allowing the PBL to directly read blocks off the
>> SD-Card without a full MMC/SD-Card driver.
> 
> I see.
> 
>>> We certainly used low
>>> capacity (like 32 MB for example) and high capacity cards (4G or more)
>>> with that system.
>>>
>>> You probably already looked for a reliable way to detect this.  I was
>>> just curious why this needs to be hardcoded.
>>
>> It's been a while since I wrote the sama5d27 PBL support (which the
>> sama5d3 support I am patching here is based on), but I recall that
>> the send op cond did not work in transfer mode, the card must be sent
>> into idle first, i.e. reset. I'd be happy if there happens to be indeed
>> a way to deduce high capacity mode without resetting and having to repeat
>> the SD-Card setup in the first boot stage.
> 
> Makes sense.  And yes, IIRC correctly, that register read happens
> while setting up the card.  If you can not or don't want to reset the
> card again, it's probably tricky.
> 
>> There are of course alternatives to this:
>>   - build barebox twice with different configs for first and second stage:
>>     There is already code for this (chainload code that uses barebox proper
>>     driver that fully re-initializes card). The first stage config needs to
>>     be small to fit into SRAM, so we can no longer generate both stages from
>>     the same multi-image config as we already do.
>>
>>   - provide full MMC support in PBL:
>>     The objective so far was to keep PBL code to the bare minimum. Doing
>>     full MMC setup there violates that.
>>
>>
>> AFAIK, all barebox platforms, except for i.MX, went for the first alternative
>> of having multiple configs. barebox on i.MX doesn't have this issue, because
>> it reads barebox from offset 0 of the SD/MMC, which works equally well whether
>> booting off default or high capacity cards.
>>
>> I like how the i.MX defconfig generates more than a hundred images in one go and
>> wanted AT91 to have something similar, but the fact that FAT needs to seek around
>> (offsets != 0) complicates this.
>> The trade off I took then was assuming high capacity cards and postpone the decision
>> on how to deal with default capacity cards in PBL into the future.
> 
> I just had the idea of just trying to read from different small
> offsets and compare it with the block you get from offset 0.  For
> example reading 500 bytes from offset 10 and reading 510 bytes from
> offset 0 in byte mode should match the last 500 bytes, but probably in
> block mode that wouldn't match.  But that's obviously highly dependent
> on the card content and more an ugly hack, right?

Tested that yesterday: reading from the 2G default capacity card at offset 1
caused a hang. So any heuristic will probably need to read 512-byte aligned
blocks.

Cheers,
Ahmad

> 
> Greets
> Alex
> 
> 

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