linux-kernel - Re: [PATCH v3 1/1] PCI: endpoint: pci-epf-test: Call pci_epf_test_raise

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Message-ID: <20240820092359.tfvmjadm6tfxyqvm@thinkpad>
Date: Tue, 20 Aug 2024 14:53:59 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Rick Wertenbroek <rick.wertenbroek@...il.com>
Cc: Damien Le Moal <dlemoal@...nel.org>, rick.wertenbroek@...g-vd.ch,
	alberto.dassatti@...g-vd.ch,
	Krzysztof Wilczyński <kw@...ux.com>,
	Kishon Vijay Abraham I <kishon@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Niklas Cassel <cassel@...nel.org>, Frank Li <Frank.Li@....com>,
	Lars-Peter Clausen <lars@...afoo.de>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/1] PCI: endpoint: pci-epf-test: Call
 pci_epf_test_raise_irq() on failed DMA check

On Tue, Aug 20, 2024 at 10:43:56AM +0200, Rick Wertenbroek wrote:
> On Tue, Aug 20, 2024 at 10:18 AM Damien Le Moal <dlemoal@...nel.org> wrote:
> >
> > On 8/20/24 16:10, Rick Wertenbroek wrote:
> > > The pci-epf-test PCI endpoint function /drivers/pci/endpoint/function/pci-epf_test.c
> > > is meant to be used in a PCI endpoint device connected to a host computer
> > > with the host side driver: /drivers/misc/pci_endpoint_test.c.
> > >
> > > The host side driver can request read/write/copy transactions from the
> > > endpoint function and expects an IRQ from the endpoint function once
> > > the read/write/copy transaction is finished. These can be issued with or
> > > without DMA enabled. If the host side driver requests a read/write/copy
> > > transaction with DMA enabled and the endpoint function does not support
> > > DMA, the endpoint would only print an error message and wait for further
> > > commands without sending an IRQ because pci_epf_test_raise_irq() is
> > > skipped in pci_epf_test_cmd_handler(). This results in the host side
> > > driver hanging indefinitely waiting for the IRQ.
> > >
> > > Call pci_epf_test_raise_irq() when a transfer with DMA is requested but
> > > DMA is unsupported. The host side driver will no longer hang but report
> > > an error on transfer (printing "NOT OKAY") thanks to the checksum because
> > > no data was moved.
> > >
> > > Clarify the error message in the endpoint function as "Cannot ..." is
> > > vague and does not state the reason why it cannot be done.
> > >
> > > Signed-off-by: Rick Wertenbroek <rick.wertenbroek@...il.com>
> > > ---
> > >  drivers/pci/endpoint/functions/pci-epf-test.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/pci/endpoint/functions/pci-epf-test.c b/drivers/pci/endpoint/functions/pci-epf-test.c
> > > index 7c2ed6eae53a..b02193cef06e 100644
> > > --- a/drivers/pci/endpoint/functions/pci-epf-test.c
> > > +++ b/drivers/pci/endpoint/functions/pci-epf-test.c
> > > @@ -649,7 +649,8 @@ static void pci_epf_test_cmd_handler(struct work_struct *work)
> > >
> > >       if ((READ_ONCE(reg->flags) & FLAG_USE_DMA) &&
> > >           !epf_test->dma_supported) {
> > > -             dev_err(dev, "Cannot transfer data using DMA\n");
> > > +             dev_err(dev, "DMA transfer not supported\n");
> >
> > Should we set the FAIL status flag here ?
> > E.g.:
> >                  reg->status |= STATUS_READ_FAIL;
> >
> > Note: I have no idea why the status flags are different for the different
> > operations. We should really have a single SUCCESS/FAIL flag common to all
> > operations. So I think we could just do:
> >
> >                 reg->status |= STATUS_READ_FAIL | STATUS_WRITE_FAIL |
> >                         STATUS_COPY_FAIL;
> >
> > here, or go back to your v1 and handle the failure in each operation function to
> > set the correct flag.
> >
> 
> Good catch, indeed with the check outside of the functions, the status
> FAIL bits are not set. I think setting the status as a combined fail
> flag makes sense, however, it conveys the idea that read/write/copy
> failed whereas only one of them actually failed.
> 
> I agree that a single status SUCCESS/FAIL flag would be simpler. But
> changing this would require changes on both sides (EP & RC) and will
> reduce compatibility between EP and RC side driver versions, so I
> would refrain from changing this.
> 

I think it is OK to change the status flags and do the right thing. If someone
reports a test failure, then we can ask them to upgrade their kernel. Given that
this this just a test application, I don't think it is a big deal.

- Mani

>  I think I still prefer the v1/v2 code because even as it has a little
> bit of duplication it is clear and sets the correct FAIL bit without
> extra logic whereas here we either set all FAIL bits or have to add
> extra logic.
> 
> Thank you for spotting this.
> 
> > > +             pci_epf_test_raise_irq(epf_test, reg);
> > >               goto reset_handler;
> > >       }
> > >
> >
> > --
> > Damien Le Moal
> > Western Digital Research
> >
> 
> Best regards,
> Rick

-- 
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