Re: [PATCH net-next] net: ipa: avoid 64-bit modulus

On 3/24/21 11:27 AM, David Laight wrote:

From: Alex Elder

quoted

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 <redacted>
---
  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

-
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