Re: [PATCH 2/2] misc: pci_endpoint_test: Add support for capabilities

[Niklas Cassel <cassel@kernel.org>]

On Wed, Nov 20, 2024 at 11:53:29AM -0500, Frank Li wrote:
> On Wed, Nov 20, 2024 at 04:57:33PM +0100, Niklas Cassel wrote:
> > If running pci_endpoint_test.c (host side) against a version of
> > pci-epf-test.c (EP side), we will not see any capabilities being set.
> >
> > For now, only add the CAP_HAS_ALIGN_ADDR capability.
> >
> > If the CAP_HAS_ALIGN_ADDR is set, that means that the EP side supports
> > reading/writing to an address without any alignment requirements.
> >
> > Thus, if CAP_HAS_ALIGN_ADDR is set, make sure that we do not add any
> > specific padding to the buffers that we allocate (which was only made
> > in order to get the buffers to satisfy certain alignment requirements).
> >
> > Signed-off-by: Niklas Cassel <cassel@kernel.org>
> > ---
> >  drivers/misc/pci_endpoint_test.c | 21 +++++++++++++++++++++
> >  1 file changed, 21 insertions(+)
> >
> > diff --git a/drivers/misc/pci_endpoint_test.c b/drivers/misc/pci_endpoint_test.c
> > index 3aaaf47fa4ee..ab2b322410fb 100644
> > --- a/drivers/misc/pci_endpoint_test.c
> > +++ b/drivers/misc/pci_endpoint_test.c
> > @@ -69,6 +69,9 @@
> >  #define PCI_ENDPOINT_TEST_FLAGS			0x2c
> >  #define FLAG_USE_DMA				BIT(0)
> >
> > +#define PCI_ENDPOINT_TEST_CAPS			0x30
> > +#define CAP_HAS_ALIGN_ADDR			BIT(0)
> > +
> >  #define PCI_DEVICE_ID_TI_AM654			0xb00c
> >  #define PCI_DEVICE_ID_TI_J7200			0xb00f
> >  #define PCI_DEVICE_ID_TI_AM64			0xb010
> > @@ -805,6 +808,22 @@ static const struct file_operations pci_endpoint_test_fops = {
> >  	.unlocked_ioctl = pci_endpoint_test_ioctl,
> >  };
> >
> > +static void pci_endpoint_test_get_capabilities(struct pci_endpoint_test *test)
> > +{
> > +	struct pci_dev *pdev = test->pdev;
> > +	struct device *dev = &pdev->dev;
> > +	u32 caps;
> > +	bool has_align_addr;
> > +
> > +	caps = pci_endpoint_test_readl(test, PCI_ENDPOINT_TEST_CAPS);
> 
> I worry about if there are problem if EP have not such register. for
> example, if reg's space size is 64, but host try to read pos 68. I think it
> is original design problem, it should have one 'version' or 'size' in
> register list.

Hello Frank,

The test BAR is allocated using pci_epf_alloc_space(), which allocates the
backing memory using dma_alloc_coherent(), which will return zeroed memory
regardless of __GFP_ZERO was set or not.

This means that running a new version of pci-endpoint-test.c (host side)
with and old version of pci-epf-test.c (EP side) will not see any
capabilities being set (as intended), so this is backwards compatible.


And as you probably know, pci-epf-test will always allocate at least 128
bytes for the test BAR:
https://github.com/torvalds/linux/blob/v6.12/drivers/pci/endpoint/functions/pci-epf-test.c#L833

This patch uses:
#define PCI_ENDPOINT_TEST_CAPS                     0x30

0x30 == 48, so we are currently only using 49 bytes.

So we should be good.


Kind regards,
Niklas