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

[<Kelvin.Cao@microchip.com>]

On Wed, 2023-10-11 at 16:36 +0000, Kelvin.Cao@microchip.com wrote:
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> On Wed, 2023-10-11 at 17:18 +0530, Vinod Koul wrote:
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> > On 10-10-23, 21:23, Kelvin.Cao@microchip.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.
> 

Hi Vinod,

I've submitted v7 (title: [PATCH v7 0/1] Switchtec Switch DMA Engine
Driver) which removed the callback support for wimm as you suggested.
Please let me know if that looks good to you. 

Thanks,
Kelvin