linux-kernel - Re: [PATCH v2 4/4] selftests: pci_endpoint: Migrate to Kselftest framework

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Message-ID: <20241129182547.prc4bsmqjvylfymk@thinkpad>
Date: Fri, 29 Nov 2024 23:55:51 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Niklas Cassel <cassel@...nel.org>
Cc: kw@...ux.com, gregkh@...uxfoundation.org, arnd@...db.de,
	lpieralisi@...nel.org, shuah@...nel.org, kishon@...nel.org,
	aman1.gupta@...sung.com, p.rajanbabu@...sung.com,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	bhelgaas@...gle.com, linux-arm-msm@...r.kernel.org, robh@...nel.org,
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 4/4] selftests: pci_endpoint: Migrate to Kselftest
 framework

On Fri, Nov 29, 2024 at 06:13:12PM +0100, Niklas Cassel wrote:
> On Fri, Nov 29, 2024 at 10:22:56PM +0530, Manivannan Sadhasivam wrote:
> > On Fri, Nov 29, 2024 at 05:42:26PM +0100, Niklas Cassel wrote:
> > > On Fri, Nov 29, 2024 at 10:05:55PM +0530, Manivannan Sadhasivam wrote:
> > > > On Fri, Nov 29, 2024 at 02:51:26PM +0100, Niklas Cassel wrote:
> > > > > Hello Mani,
> > > > > 
> > > > > On Fri, Nov 29, 2024 at 02:54:15PM +0530, Manivannan Sadhasivam wrote:
> > > > > > Migrate the PCI endpoint test to Kselftest framework. All the tests that
> > > > > > were part of the previous pcitest.sh file were migrated.
> > > > > > 
> > > > > > Below is the exclusive list of tests:
> > > > > > 
> > > > > > 1. BAR Tests (BAR0 to BAR5)
> > > > > > 2. Legacy IRQ Tests
> > > > > > 3. MSI Interrupt Tests (MSI1 to MSI32)
> > > > > > 4. MSI-X Interrupt Tests (MSI-X1 to MSI-X2048)
> > > > > > 5. Read Tests - MEMCPY (For 1, 1024, 1025, 1024000, 1024001 Bytes)
> > > > > > 6. Write Tests - MEMCPY (For 1, 1024, 1025, 1024000, 1024001 Bytes)
> > > > > > 7. Copy Tests - MEMCPY (For 1, 1024, 1025, 1024000, 1024001 Bytes)
> > > > > > 8. Read Tests - DMA (For 1, 1024, 1025, 1024000, 1024001 Bytes)
> > > > > > 9. Write Tests - DMA (For 1, 1024, 1025, 1024000, 1024001 Bytes)
> > > > > > 10. Copy Tests - DMA (For 1, 1024, 1025, 1024000, 1024001 Bytes)
> > > > > 
> > > > > I'm not sure if it is a great idea to add test case number 10.
> > > > > 
> > > > > While it will work if you use the "dummy memcpy" DMA channel which uses
> > > > > MMIO under the hood, if you actually enable a real DMA controller (which
> > > > > often sets the DMA_PRIVATE cap in the DMA controller driver (e.g. if you
> > > > > are using a DWC based PCIe EP controller and select CONFIG_DW_EDMA=y)),
> > > > > pci_epf_test_copy() will fail with:
> > > > > [   93.779444] pci_epf_test pci_epf_test.0: Cannot transfer data using DMA
> > > > > 
> > > > 
> > > > So the idea is to exercise all the options provided by the epf-test driver. In
> > > > that sense, we need to have the DMA COPY test. However, I do agree that the
> > > > common DMA controllers will fail this case. So how about just simulating the DMA
> > > > COPY for controllers implementing DMA_PRIVATE cap? I don't think it hurts to
> > > > have this feature in test driver.
> > > 
> > > I guess you could modify pci-epf-test to simply do MMIO in test_copy(),
> > > if USE_DMA && DMA_PRIVATE is set, as you suggest.
> > > 
> > 
> > No not memcpy, but using the DMA to copy from src to local buf and then local
> > buf to dst. This way, we do not need to fallback and at the same time simulate
> > DMA COPY.
> 
> Sounds very slow :)
> 
> What would be the value to add such code to pci-epf-test?
> 

Well, the test case is to test COPY functionality using DMA. Either we use
MEM_TO_MEM if supported, or just do DMA from source to dst. Even if the
performance is going to be half of what read/write would achieve separately, it
would give users a real benchmark. Otherwise, we have to skip the test case
altogether. Like,

./pci_endpoint_test -f pci_ep_basic -v memcpy -T COPY_TEST -v dma

Perhaps we should document this limitation and show above command to skip the
COPY_TEST for DMA?

> Sounds like we would just add a lot of extra code in pci-epf-test.c that
> would not test anything new. (It would basically just be the DMA read test
> followed by the DMA write test. If those tests pass, this new simulated
> test should be guaranteed to pass.)
> 
> Wouldn't it make more sense to simply do something like:
> 
> if (use_dma && dma_prive) {
> 	dev_warn(dev, "DEV_TO_DEV not supported with USE_DMA, falling back to MMIO\n");
> 	use_dma = 0;
> }
> 

Maybe yes, but memcpy is also doing the same. The problem with falling back is
that, it provides a fake benchmark to the users which I want to avoid doing so.

- Mani

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