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

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Date:   Wed, 24 Mar 2021 12:07:27 -0500
From:   Alex Elder <elder@...e.org>
To:     David Laight <David.Laight@...LAB.COM>,
        'Alex Elder' <elder@...aro.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>
Cc:     "rdunlap@...radead.org" <rdunlap@...radead.org>,
        "bjorn.andersson@...aro.org" <bjorn.andersson@...aro.org>,
        "evgreen@...omium.org" <evgreen@...omium.org>,
        "cpratapa@...eaurora.org" <cpratapa@...eaurora.org>,
        "subashab@...eaurora.org" <subashab@...eaurora.org>,
        "elder@...nel.org" <elder@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] net: ipa: avoid 64-bit modulus

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@...radead.org>
>> Signed-off-by: Alex Elder <elder@...aro.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
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