linux-kernel - Re: [PATCH 4/4] csky: Add dma_inv_range for DMA_FROM

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Date:   Tue, 30 Jul 2019 23:48:03 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-csky@...r.kernel.org, feng_shizhu@...uatech.com,
        zhang_jian5@...uatech.com, zheng_xingjian@...uatech.com,
        zhu_peng@...uatech.com, Guo Ren <ren_guo@...ky.com>
Subject: Re: [PATCH 4/4] csky: Add dma_inv_range for DMA_FROM_DEVICE

Thx Arnd,

On Tue, Jul 30, 2019 at 11:22 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Tue, Jul 30, 2019 at 5:11 PM Guo Ren <guoren@...nel.org> wrote:
> > > > diff --git a/arch/csky/mm/dma-mapping.c b/arch/csky/mm/dma-mapping.c
> > > >                 cache_op(paddr, size, dma_wb_range);
> > > >                 break;
> > > >         case DMA_FROM_DEVICE:
> > > > +               cache_op(paddr, size, dma_inv_range);
> > > > +               break;
> > > >         case DMA_BIDIRECTIONAL:
> > > >                 cache_op(paddr, size, dma_wbinv_range);
> > > >                 break;
> > >
> > > When syncing 'for_cpu', you should not need to write back, because
> > > there won't be any dirty cache lines.
> >
> > I just follow the dma_data_direction param, seems dir param and
> > function are a little bit duplicated. And our cpu won't clear clean
> > cache line into memory, so dma_wb_page won't cause problem.
> > Seems arch_sync_dma_for_cpu with dir=DMA_TO_DEVICE is
> > self-contradictory.
>
> Right, you generally don't need to do cache management for that
> combination.
>
> There might be other things to do here though, e.g. with a strict
> iommu implementation one could tear down the i/o page table
> entry to prevent the device from accessing a buffer while that is
> owned by the cpu.
Tear down i/o page table shouldn't be put here and it's for map/unmap().
And I think arch_sync_dma_for_cpu/device should only do cache issues.

--
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/