linux-kernel - Re: [PATCH v4 5/7] powerpc/iommu: Move iommu_table cleaning routine to iommu_table

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Date:   Wed, 22 Jul 2020 20:45:25 -0300
From:   Leonardo Bras <leobras.c@...il.com>
To:     Brian King <brking@...ux.vnet.ibm.com>,
        Alexey Kardashevskiy <aik@...abs.ru>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Joel Stanley <joel@....id.au>,
        Christophe Leroy <christophe.leroy@....fr>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Ram Pai <linuxram@...ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 5/7] powerpc/iommu: Move iommu_table cleaning routine
 to iommu_table_clean

On Tue, 2020-07-21 at 19:52 -0500, Brian King wrote:
> > 
> > As of today, there seems to be nothing like that happening in the
> > driver I am testing. 
> > I spoke to Brian King on slack, and he mentioned that at the point DDW
> > is created there should be no allocations in place.
> 
> I think there are a couple of scenarios here. One is where there is a DMA
> allocation prior to a call to set the DMA mask. Second scenario is if the
> driver makes multiple calls to set the DMA mask. I would argue that a properly
> written driver should tell the IOMMU subsystem what DMA mask it supports prior
> to allocating DMA memroy. Documentation/core-api/dma-api-howto.rst should
> describe what is legal and what is not.
> 
> It might be reasonable to declare its not allowed to allocate DMA memory
> and then later change the DMA mask and clearly call this out in the documentation
> if its not already.
> 
> -Brian

Thank you for the feedback Brian!

That makes sense to me. I will try to have this in mind for the next
patchset. 

Best regards,