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 > > -- Pengutronix e.K. | | Steuerwalder Str. 21 | http://www.pengutronix.de/ | 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 | _______________________________________________ barebox mailing list barebox@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/barebox
