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

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Message-ID: <ZiYuIaX7ZV0exKMt@ryzen>
Date: Mon, 22 Apr 2024 11:30:09 +0200
From: Niklas Cassel <cassel@...nel.org>
To: Manivannan Sadhasivam <mani@...nel.org>
Cc: Vinod Koul <vkoul@...nel.org>,
	Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
	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, Damien Le Moal <dlemoal@...nel.org>,
	Frank Li <Frank.Li@....com>
Subject: Re: [PATCH 05/11] PCI: epf-{mhi/test}: Move DMA initialization to
 EPC init callback

On Mon, Apr 22, 2024 at 01:25:21PM +0530, Manivannan Sadhasivam wrote:
> > 
> > What I would like is more consistency between the EPF drivers.
> > 
> > I guess an if-statement that skips the pci_epc_map_addr() in pci-epf-test
> > if using eDMA would make pci-epf-mhi and pci-epf-test most consistent.
> > 
> 
> Agree.


> > 1) Do we want to rely on the fact that hopefully none of the iATUs in the DWC
> > controller has configured a mapping that might mess things up for us?
> > I don't see why the PCI/DMA address of the remote buffer, supplied to
> > pci-epf-test via test_reg BAR, might not fall within the physical iATU window
> > on the local EP system. (As long as the PCI EPF driver has mapped any address
> > using pci_epc_map_addr().)
> > 
> > This is a big argument that EPF drivers running on a DWC-based EPC should
> > definitely NOT call pci_epc_map_addr() needlessly when using eDMA, as it
> > can be catastrophic. (pci-epf-test needs to be patched.)
> > 
> 
> Right. There is no need to do iATU translation for DMA. I avoid that in MHI
> driver.

There is no need for pci_epc_map_addr() when using DMA_SLAVE *for DWC-based
controllers*.

Are we certain that this will not break pci-epf-test for non DWC-based
controllers?


> > 2) Can we really assume that both pci-epf-test and pci-epf-mhi does not need
> > to call pci_epc_map_addr() when using a DMA_SLAVE DMA controller?
> > This seems to be designed only with DWC in mind. Other PCIe endpoint
> > controllers might require this.
> > (Yes, for DWC-based controllers, this definitely should be skipped, but EPF
> > drivers are supposed to be independent from a specific EPC.)
> > 
> 
> For TEST yes, but for MHI, no. In MHI, I kind of mix both iATU and DMA to ripe
> most of the performance (small vs big transactions). But for the TEST driver, it
> is fair to not call pci_epc_map_addr() when DMA_SLAVE is supported.

I agree that we should definitely skip pci_epc_map_addr() in pci-epf-test when
using DMA_SLAVE on DWC-based controllers, but I don't feel comfortable in
submitting a patch that does this unconditionally for pci-epf-test.c,
as I don't know how the DMA hardware in:
drivers/pci/controller/cadence/pcie-cadence-ep.c
drivers/pci/controller/pcie-rcar-ep.c
drivers/pci/controller/pcie-rockchip-ep.c

works, and I do not want to regress them.

I did suggest that DWC-based drivers could set a DMA_SLAVE_SKIP_MEM_MAP flag
or similar when registering the eDMA, which pci-epf-test then could check,
but I got no response if anoyone else thought that this was a good idea.


Kind regards,
Niklas