Re: [PATCH v2 3/9] mmc: cqhci: initialize upper 64 bits of 128-bit task descriptors

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Dec 03, 2020 at 08:45:15AM +0200, Adrian Hunter wrote:
> On 3/12/20 4:05 am, Eric Biggers wrote:
> > From: Eric Biggers <ebiggers@google.com>
> > 
> > Move the task descriptor initialization into cqhci_prep_task_desc(), and
> > make it initialize all 128 bits of the task descriptor if the host
> > controller is using 128-bit task descriptors.
> > 
> > This is needed to prepare for CQHCI inline encryption support, which
> > requires 128-bit task descriptors and uses the upper 64 bits.
> > 
> > Note: since some host controllers already enable 128-bit task
> > descriptors, it's unclear why the previous code worked when it wasn't
> > initializing the upper 64 bits.  One possibility is that the bits are
> > being ignored because the features that use them aren't enabled yet.
> > In any case, setting them to 0 won't hurt.
> 
> Coherent allocations are zero-initialized.  So the upper 64-bits stay zero.
> People set 128-bit anyway because the hardware needs it.

Okay, that explains it then -- I didn't realize that dma_alloc_coherent() always
returns zeroed memory.  It isn't mentioned in
Documentation/core-api/dma-api.rst, and there's no kerneldoc comment, so it
wasn't clear.  But apparently it's intentional; see commit 518a2f1925c3
("dma-mapping: zero memory returned from dma_alloc_*").

I'll fix this commit message in the next version.

- Eric