linux-kernel - Re: [PATCH v6 1/1] dmaengine: switchtec-dma: Introduce Switchtec DMA engine PCI driver

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Message-ID: <f6beb06cc707329923cc460545afd6cfe9fa065d.camel@microchip.com>
Date:   Wed, 11 Oct 2023 16:36:57 +0000
From:   <Kelvin.Cao@...rochip.com>
To:     <vkoul@...nel.org>
CC:     <dmaengine@...r.kernel.org>, <George.Ge@...rochip.com>,
        <hch@...radead.org>, <linux-kernel@...r.kernel.org>,
        <logang@...tatee.com>, <christophe.jaillet@...adoo.fr>
Subject: Re: [PATCH v6 1/1] dmaengine: switchtec-dma: Introduce Switchtec DMA
 engine PCI driver

On Wed, 2023-10-11 at 17:18 +0530, Vinod Koul wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you
> know the content is safe
> 
> On 10-10-23, 21:23, Kelvin.Cao@...rochip.com wrote:
> > On Mon, 2023-10-09 at 11:08 +0530, Vinod Koul wrote:
> 
> > > > u64 size_to_transfer;
> > > 
> > > Why cant the client driver write to doorbell, is there anything
> > > which
> > > prevents us from doing so?
> > 
> > I think the potential challenge here for the client driver to ring
> > db
> > is that the client driver (host RC) is a different requester in the
> > PCIe hierarchy compared to DMA EP, in which case PCIe ordering need
> > to
> > be considered.
> > 
> > As PCIe ensures that reads don't pass writes, we can insert a read
> > DMA
> > operation with DMA_PREP_FENSE flag in between the two DMA writes
> > (one
> > for data transfer and one for notification) to ensure the ordering
> > for
> > the same requester DMA EP. I'm not sure if the RC could ensure the
> > same
> > ordering if the client driver issue MMIO write to db after the data
> > DMA
> > and read DMA completion, so that the consumer is guaranteed the
> > transferred data is ready in memory when the db is triggered by the
> > client MMIO write. I guess it's still doable with MMIO write but
> > just
> > some special consideration needed.
> 
> Given that it is a single value, overhead of doing a new txn would be
> higher than a mmio write! I think that should be preferred
> 
> --

Ok. I'll remove the callback and come up with v7. Thank you Vinod for
your comments.

Kelvin