Messages in this thread | | | Subject | Re: [PATCH net-next] net: ipa: avoid 64-bit modulus | From | Alex Elder <> | Date | Wed, 24 Mar 2021 12:07:27 -0500 |
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On 3/24/21 11:27 AM, David Laight wrote: > From: Alex Elder >> Sent: 23 March 2021 01:05 >> It is possible for a 32 bit x86 build to use a 64 bit DMA address. >> >> There are two remaining spots where the IPA driver does a modulo >> operation to check alignment of a DMA address, and under certain >> conditions this can lead to a build error on i386 (at least). >> >> The alignment checks we're doing are for power-of-2 values, and this >> means the lower 32 bits of the DMA address can be used. This ensures >> both operands to the modulo operator are 32 bits wide. >> >> Reported-by: Randy Dunlap <rdunlap@infradead.org> >> Signed-off-by: Alex Elder <elder@linaro.org> >> --- >> drivers/net/ipa/gsi.c | 11 +++++++---- >> drivers/net/ipa/ipa_table.c | 9 ++++++--- >> 2 files changed, 13 insertions(+), 7 deletions(-) >> >> diff --git a/drivers/net/ipa/gsi.c b/drivers/net/ipa/gsi.c >> index 7f3e338ca7a72..b6355827bf900 100644 >> --- a/drivers/net/ipa/gsi.c >> +++ b/drivers/net/ipa/gsi.c >> @@ -1436,15 +1436,18 @@ static void gsi_evt_ring_rx_update(struct gsi_evt_ring *evt_ring, u32 index) >> /* Initialize a ring, including allocating DMA memory for its entries */ >> static int gsi_ring_alloc(struct gsi *gsi, struct gsi_ring *ring, u32 count) >> { >> - size_t size = count * GSI_RING_ELEMENT_SIZE; >> + u32 size = count * GSI_RING_ELEMENT_SIZE; >> struct device *dev = gsi->dev; >> dma_addr_t addr; >> >> - /* Hardware requires a 2^n ring size, with alignment equal to size */ >> + /* Hardware requires a 2^n ring size, with alignment equal to size. >> + * The size is a power of 2, so we can check alignment using just >> + * the bottom 32 bits for a DMA address of any size. >> + */ >> ring->virt = dma_alloc_coherent(dev, size, &addr, GFP_KERNEL); > > Doesn't dma_alloc_coherent() guarantee that alignment? > I doubt anywhere else checks?
I normally wouldn't check something like this if it weren't guaranteed. I'm not sure why I did it here.
I see it's "guaranteed to be aligned to the smallest PAGE_SIZE order which is greater than or equal to the requested size." So I think the answer to your question is "yes, it does guarantee that."
I'll make a note to remove this check in a future patch, and will credit you with the suggestion.
Thanks.
-Alex
> > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) >
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