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 13:56:36 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Niklas Cassel <cassel@...nel.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 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.

- Mani

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