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Re: [RFC PATCH 9/9] hw/sd: Allow card size not power of 2 again

From: Warner Losh
Subject: Re: [RFC PATCH 9/9] hw/sd: Allow card size not power of 2 again
Date: Thu, 24 Jun 2021 10:08:42 -0600

> On Jun 24, 2021, at 4:56 AM, Peter Maydell <peter.maydell@linaro.org> wrote:
> On Thu, 24 Jun 2021 at 11:27, Tom Yan <tom.ty89@gmail.com> wrote:
>> I really think we should get (/ have gotten) things clear first. What
>> exactly is the bug we have been talking about here? I mean like, where
>> does it occur and what's the nature of it.
>> 1. Is it specific to a certain type / model of backend / physical
>> storage device that will be made use of by qemu for the emulated
>> storage? (I presume not since you mention about image, unless you
>> irrationally related/bound the emulated storage type and the physical
>> storage type together.)
>> 2. Does it have anything to do with a certain flaw in qemu itself?
>> Like the code that does read/write operation is flawed that it cannot
>> be handled by a certain *proper* backend device?
>> 3. Or is it actually a bug in a certain driver / firmware blob that
>> will be used by an *emulated* device in the guest? In that case, can
>> we emulate another model so that it won't be using the problematic
>> driver / firmware?
> Definitely agreed -- before we start changing QEMU code we need
> to identify clearly (a) what the real hardware does and (b) what
> the situation was we were originally trying to fix.

Real SD and MMC cards do not have power-of-two constraints in the
number of blocks. None of the SD/MMC bridges I’ve ever dealt with
have a power-of-two constraint on the size of the card. The only
constraint on the size related to the card is the block size.

If non-power-of-2 sized cards fail, that’s a cat-1 sev-1 bug that needs
to be fixed, rather than a warning be generated.


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