linux-kernel - Re: [PATCH 05/11] PCI: epf-{mhi/test}: Move DMA initialization to EPC init callback

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Date: Tue, 26 Mar 2024 15:27:27 +0100
From: Niklas Cassel <cassel@...nel.org>
To: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc: Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Kishon Vijay Abraham I <kishon@...nel.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Jonathan Hunter <jonathanh@...dia.com>,
	Jingoo Han <jingoohan1@...il.com>,
	Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
	linux-pci@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-kernel@...r.kernel.org, mhi@...ts.linux.dev,
	linux-tegra@...r.kernel.org
Subject: Re: [PATCH 05/11] PCI: epf-{mhi/test}: Move DMA initialization to
 EPC init callback

On Tue, Mar 26, 2024 at 12:05:42PM +0100, Niklas Cassel wrote:
> On Tue, Mar 26, 2024 at 01:56:36PM +0530, Manivannan Sadhasivam wrote:
> > On Fri, Mar 22, 2024 at 05:10:06PM +0100, Niklas Cassel wrote:
> > > On Thu, Mar 14, 2024 at 08:53:44PM +0530, Manivannan Sadhasivam wrote:
> > > > To maintain uniformity across EPF drivers, let's move the DMA
> > > > initialization to EPC init callback. This will also allow us to deinit DMA
> > > > during PERST# assert in the further commits.
> > > > 
> > > > For EPC drivers without PERST#, DMA deinit will only happen during driver
> > > > unbind.
> > > > 
> > > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> > > > ---
> > > 
> > > Reviewed-by: Niklas Cassel <cassel@...nel.org>
> > > 
> > > 
> > > For the record, I was debugging a problem related to EPF DMA recently
> > > and was dumping the DMA mask for the struct device of the epf driver.
> > > I was a bit confused to see it as 32-bits, even though the EPC driver
> > > has it set to 64-bits.
> > > 
> > > The current code works, because e.g., pci_epf_test_write(), etc,
> > > does:
> > > struct device *dma_dev = epf->epc->dev.parent;
> > > dma_map_single(dma_dev, ...);
> > > 
> > > but it also means that all EPF drivers will do this uglyness.
> > > 
> > 
> > This ugliness is required as long as the dmaengine is associated only with the
> > EPC.
> > 
> > > 
> > > 
> > > However, if a EPF driver does e.g.
> > > dma_alloc_coherent(), and sends in the struct *device for the EPF,
> > > which is the most logical thing to do IMO, it will use the wrong DMA
> > > mask.
> > > 
> > > Perhaps EPF or EPC code should make sure that the struct *device
> > > for the EPF will get the same DMA mask as epf->epc->dev.parent,
> > > so that EPF driver developer can use the struct *epf when calling
> > > e.g. dma_alloc_coherent().
> > > 
> > 
> > Makes sense. I think it can be done during bind() in the EPC core. Feel free to
> > submit a patch if you like, otherwise I'll keep it in my todo list.
> 
> So we still want to test:
> -DMA API using the eDMA
> -DMA API using the "dummy" memcpy dma-channel.
> 
> However, it seems like both pci-epf-mhi.c and pci-epf-test.c
> do either:
> -Use DMA API
> or
> -Use memcpy_fromio()/memcpy_toio() instead of DMA API
> 
> 
> To me, it seems like we should always be able to use
> DMA API (using either a eDMA or "dummy" memcpy).
> 
> I don't really see the need to have the path that does:
> memcpy_fromio()/memcpy_toio().
> 
> I know that for DWC, when using memcpy (and this also
> memcpy via DMA API), we need to map the address using
> iATU first.
> 
> But that could probably be done using another flag,
> perhaps rename that flag FLAG_USE_DMA to NEEDS_MAP or
> something.
> (Such that we can change these drivers to only have a
> code path that uses DMA API.)

Looking at pci-epf-mhi.c, it seems to use names like:
pci_epf_mhi_iatu_read() and pci_epf_mhi_edma_read().

This seems to be a very DWC focused naming.

AFAICT, EPF drivers should work on any PCIe EP controller that implements
the EPC API.

Yes, I understand that it is only Qualcomm that uses this MHI interface/bus,
but what is stopping Qualcomm from using a non-DWC based PCIe EP controller
in an upcoming SoC?

Surely that Qualcomm SoC could still implement the MHI interface/bus,
so perhaps the naming in this EPF driver should use somewhat less
"EPC vendor specific" function names?


Kind regards,
Niklas