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

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Date:   Mon, 5 Aug 2019 23:51:15 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Guo Ren <guoren@...nel.org>,
        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

On Tue, Jul 30, 2019 at 03:43:20PM +0200, Arnd Bergmann wrote:
> When syncing 'for_cpu', you should not need to write back, because
> there won't be any dirty cache lines.
> 
> If you have a CPU core that does not do speculative loads, you also don't
> need to invalidate here, because you have already done that in the
> _for_device() case, the only reason to invalidate the CPU cache
> again is if a speculative load created a stale cache line that now
> shadows the data received from the device.

Yes.  And that is one reason why I want to lift a set of common helpers
for both the speculating and non-speculating case to the common code
that just calls arch specific writeback/invalidate/writeback+invalidate
helpers.  It hasn't been a priotity so far, but maybe it becomes one
now.  Especially if I could draft someone else to help with it :)