linux-kernel - Re: [PATCH] tpm: ibmvtpm: Make use of dma_alloc

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Date:   Tue, 12 Oct 2021 20:40:32 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Cai Huoqing <caihuoqing@...du.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Peter Huewe <peterhuewe@....de>, linuxppc-dev@...ts.ozlabs.org,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tpm: ibmvtpm: Make use of dma_alloc_coherent()

On Tue, 2021-10-12 at 12:43 -0300, Jason Gunthorpe wrote:
> On Tue, Oct 12, 2021 at 06:29:58PM +0300, Jarkko Sakkinen wrote:
> > On Mon, 2021-10-11 at 00:01 +0800, Cai Huoqing wrote:
> > > Replacing kmalloc/kfree/get_zeroed_page/free_page/dma_map_single/
> >   ~~~~~~~~~
> >   Replace
> > 
> > > dma_unmap_single() with dma_alloc_coherent/dma_free_coherent()
> > > helps to reduce code size, and simplify the code, and coherent
> > > DMA will not clear the cache every time.
> > > 
> > > Signed-off-by: Cai Huoqing <caihuoqing@...du.com>
> > 
> > If this does not do functionally anything useful, there's no
> > reason to apply this.
> 
> At least in this case it looks like the ibmvtpm is not using the DMA
> API properly. There is no sync after each data transfer. Replacing
> this wrong usage with the coherent API is reasonable.

Thank you. As long as this is documented to the commit message,
I'm cool with the change itself.

E.g. something like this would be perfectly fine replacement for the
current commit message:

"The current usage pattern for the DMA API is inappropriate, as
data transfers are not synced. Replace the existing DMA code
with the coherent DMA API."

/Jarkko