Re: [PATCH v2 1/8] hw/sd/sdcard: Fix size check for backing block image

["Jan Lübbe" <jlu@pengutronix.de>]

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.
>
> 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).

Regards,
Jan


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