linux-kernel - Re: [PATCH v2 1/1] dmaengine: switchtec-dma: Introduce Switchtec DMA engine PCI driver

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Message-ID: <ZDjpjzj3xPaeeE1c@infradead.org>
Date:   Thu, 13 Apr 2023 22:50:07 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Kelvin.Cao@...rochip.com
Cc:     hch@...radead.org, dmaengine@...r.kernel.org, vkoul@...nel.org,
        George.Ge@...rochip.com, linux-kernel@...r.kernel.org,
        logang@...tatee.com
Subject: Re: [PATCH v2 1/1] dmaengine: switchtec-dma: Introduce Switchtec DMA
 engine PCI driver

On Thu, Apr 13, 2023 at 10:40:41PM +0000, Kelvin.Cao@...rochip.com wrote:
> > > Why is the lock needed while reading the status and waiting
> > > for it with long delays?
> There's (low) chance of access to the same ctrl register from other
> paths which might change the value of status in an unexpected way. It
> also prevents the hardware operation from being interrupted until it
> indicates it has finished by a bit set in the status register.

Well, the lock is obviously required to protecte the reads to the
register.  But why do you need to hold the lock over the reads and
the delay?