lkml.org 
[lkml]   [2021]   [Mar]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH net-next] net: ipa: avoid 64-bit modulus
From
Date
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)
>

\
 
 \ /
  Last update: 2021-03-24 18:12    [W:0.066 / U:0.452 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site