Re: [PATCH V7 9/9] PCI/P2PDMA: Add a 10-Bit Tag check in P2PDMA

Bjorn Helgaas
2021-08-09T17:31:17Z

On Sat, Aug 07, 2021 at 03:11:34PM +0800, Dongdong Liu wrote:
>
> On 2021/8/6 2:12, Bjorn Helgaas wrote:
> > On Wed, Aug 04, 2021 at 09:47:08PM +0800, Dongdong Liu wrote:
> > > Add a 10-Bit Tag check in the P2PDMA code to ensure that a device with
> > > 10-Bit Tag Requester doesn't interact with a device that does not
> > > support 10-BIT Tag Completer. Before that happens, the kernel should
> > > emit a warning. "echo 0 > /sys/bus/pci/devices/.../10bit_tag" to
> > > disable 10-BIT Tag Requester for PF device.
> > > "echo 0 > /sys/bus/pci/devices/.../sriov_vf_10bit_tag_ctl" to disable
> > > 10-BIT Tag Requester for VF device.
> >
> > s/10-BIT/10-Bit/ several times.
> Will fix.
> >
> > Add blank lines between paragraphs.
> Will fix.
> >
> > > Signed-off-by: Dongdong Liu <liudongdong3@huawei.com>
> > > ---
> > > drivers/pci/p2pdma.c | 40 ++++++++++++++++++++++++++++++++++++++++
> > > 1 file changed, 40 insertions(+)
> > >
> > > diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> > > index 50cdde3..948f2be 100644
> > > --- a/drivers/pci/p2pdma.c
> > > +++ b/drivers/pci/p2pdma.c
> > > @@ -19,6 +19,7 @@
> > > #include <linux/random.h>
> > > #include <linux/seq_buf.h>
> > > #include <linux/xarray.h>
> > > +#include "pci.h"
> > >
> > > enum pci_p2pdma_map_type {
> > > PCI_P2PDMA_MAP_UNKNOWN = 0,
> > > @@ -410,6 +411,41 @@ static unsigned long map_types_idx(struct pci_dev *client)
> > > (client->bus->number << 8) | client->devfn;
> > > }
> > >
> > > +static bool check_10bit_tags_vaild(struct pci_dev *a, struct pci_dev *b,
> >
> > s/vaild/valid/
> >
> > Or maybe s/valid/safe/ or s/valid/supported/, since "valid" isn't
> > quite the right word here. We want to know whether the source is
> > enabled to generate 10-bit tags, and if so, whether the destination
> > can handle them.
> >
> > "if (check_10bit_tags_valid())" does not make sense because
> > "check_10bit_tags_valid()" is not a question with a yes/no answer.
> >
> > "10bit_tags_valid()" *might* be, because "if (10bit_tags_valid())"
> > makes sense. But I don't think you can start with a digit.
> >
> > Or maybe you want to invert the sense, e.g.,
> > "10bit_tags_unsupported()", since that avoids negation at the caller:
> >
> > if (10bit_tags_unsupported(a, b) ||
> > 10bit_tags_unsupported(b, a))
> > map_type = PCI_P2PDMA_MAP_NOT_SUPPORTED;
> Good suggestion. add a pci_ prefix.
>
> if (pci_10bit_tags_unsupported(a, b) ||
> pci_10bit_tags_unsupported(b, a))
> map_type = PCI_P2PDMA_MAP_NOT_SUPPORTED;

This treats both directions as equally important. I don't know P2PDMA
very well, but that doesn't seem like it would necessarily be the
case. I would think a common case would be device A doing DMA to B,
but B *not* doing DMA to A. So can you tell which direction you're
setting up here, and can you take advantage of any asymmetry, e.g., by
enabling 10-bit tags in the direction that supports it even if the
other direction does not?