On 02.09.25 19:07, Warner Losh wrote: > > > On Tue, Sep 2, 2025 at 10:49 AM Jan Lübbe <j...@pengutronix.de > <mailto:j...@pengutronix.de>> wrote: > > On Tue, 2025-09-02 at 18:39 +0200, Jan Kiszka wrote: > > > > I expect us to be safe and able to deal with non-pow2 regions > if we use > > > > QEMUSGList from the "system/dma.h" API. But this is a rework > nobody had > > > > time to do so far. > > > > > > We have to tell two things apart: partitions sizes on the one > side and > > > backing storage sizes. The partitions sizes are (to my reading) > clearly > > > defined in the spec, and the user partition (alone!) has to be > power of > > > 2. The boot and RPMB partitions are multiples of 128K. The sum > of them > > > all is nowhere limited to power of 2 or even only multiples of 128K. > > > > > > > Re-reading the part of the device capacity, the rules are more > complex: > > - power of two up to 2 GB > > - multiple of 512 bytes beyond that > > > > So that power-of-two enforcement was and still is likely too strict. > > > It is. Version 0 (and MMC) cards had the capacity encoded like so: > m = mmc_get_bits(raw_csd, 128, 62, 12); > e = mmc_get_bits(raw_csd, 128, 47, 3); > csd->capacity = ((1 + m) << (e + 2)) * csd->read_bl_len; > so any card less than 2GB (well, technically 4GB, but 4GB version 0 > cards were > rare and broke some stacks... I have one and I love it on my embedded > ARM board > that can't do version 1 cards). Version 1 cards encoded it like: > csd->capacity = ((uint64_t)mmc_get_bits(raw_csd, 128, > 48, 22) + > 1) * 512 * 1024; > So it's a multiple of 512k. These are also called 'high capacity' cards. > > Version 4 introduces an extended CSD, which had a pure sector count in > the EXT CSD. I think this > is only for MMC cards. And also the partition information. > > > > But I still see no indication, neither in the existing eMMC code > of QEMU > > nor the spec, that the boot and RPMB partition sizes are included > in that. > > Correct. Non-power-of-two sizes are very common for real eMMCs. > Taking a random > one from our lab: > [ 1.220588] mmcblk1: mmc1:0001 S0J56X 14.8 GiB > [ 1.228055] mmcblk1: p1 p2 p3 p4 > [ 1.230375] mmcblk1boot0: mmc1:0001 S0J56X 31.5 MiB > [ 1.233651] mmcblk1boot1: mmc1:0001 S0J56X 31.5 MiB > [ 1.236682] mmcblk1rpmb: mmc1:0001 S0J56X 4.00 MiB, chardev (244:0) > > For eMMCs using MLC NAND, you can also configure part of the user > data area to > be pSLC (pseudo single level cell), which changes the available > capacity (after > a required power cycle). > > > Yes. Extended partitions are a feature of version 4 cards, so don't have > power-of-2 limits since they are a pure sector count in the ext_csd. >
JESD84-B51A (eMMC 5.1A): "The C_SIZE parameter is used to compute the device capacity for devices up to 2 GB of density. See 7.4.52, SEC_COUNT [215:212] , for details on calculating densities greater than 2 GB." So I would now continue to enforce power-of-2 for 2G (including) cards, and relax to multiples of 512 for larger ones. Jan -- Siemens AG, Foundational Technologies Linux Expert Center
