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

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Date:   Tue, 12 Oct 2021 12:43:25 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Jarkko Sakkinen <jarkko@...nel.org>
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, 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.

Jason