Re: [PATCH net 3/4] net: ipa: use the right accessor in ipa_endpoint_status_skip()

[Alex Elder]

On 2/1/21 6:02 PM, Amy Parker wrote:
> On Mon, Feb 1, 2021 at 3:32 PM Alex Elder <elder@xxxxxxxxxx> wrote:
> > When extracting the destination endpoint ID from the status in
> > ipa_endpoint_status_skip(), u32_get_bits() is used.  This happens to
> > work, but it's wrong: the structure field is only 8 bits wide
> > instead of 32.
> >
> > Fix this by using u8_get_bits() to get the destination endpoint ID.
>
> Isn't

(I saw your second message.)

> > Signed-off-by: Alex Elder <elder@xxxxxxxxxx>
> > ---
> >   drivers/net/ipa/ipa_endpoint.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/ipa/ipa_endpoint.c b/drivers/net/ipa/ipa_endpoint.c
> > index 448d89da1e456..612afece303f3 100644
> > --- a/drivers/net/ipa/ipa_endpoint.c
> > +++ b/drivers/net/ipa/ipa_endpoint.c
> > @@ -1164,8 +1164,8 @@ static bool ipa_endpoint_status_skip(struct ipa_endpoint *endpoint,
> >                  return true;
>
> A few lines above this, endpoint_id is initialized as u32. If we're
> going for "correctness", endpoint_id should be a u8. But of course,
> this would contrast with ipa_endpoint having it as a u32.

You are correct, endpoint_id is *defined* as type u32.

But the issue here is that the field status->endp_dst_idx
has type u8.  u32_get_bits() assumes the field it is
passed has type u32; while u8_get_bits() takes a u8.

The return value of u8_get_bits() is u8, as you might
suspect.  The C standard guarantees that the u8 value
will be promoted to the u32 target type here.

> >          if (!status->pkt_len)
> >                  return true;
> > -       endpoint_id = u32_get_bits(status->endp_dst_idx,
> > -                                  IPA_STATUS_DST_IDX_FMASK);
> > +       endpoint_id = u8_get_bits(status->endp_dst_idx,
> > +                                 IPA_STATUS_DST_IDX_FMASK);
> >          if (endpoint_id != endpoint->endpoint_id)
> >                  return true;
> >
> > --
> > 2.27.0
>
> As far as I see it, using u32_get_bits instead of u8_get_bits simply
> eliminates confusion about the type of endpoint_id. Perhaps instead of
> this patch, send a patch with a comment that while u32_get_bits is
> used, the field is only 8 bits?

No.  We really want to extract a sub-field from the u8
value passed to u8_get_bits() (not u32_get_bits()).

Does that make sense?

					-Alex


> Best regards,
> Amy Parker
> (she/her/hers)