Re: [PATCH v2] scsi: 3w-9xxx: Fix endianness issues found by sparse

[Samuel Holland <samuel@sholland.org>]

On 8/5/20 2:17 AM, Arnd Bergmann wrote:
> On Wed, Aug 5, 2020 at 3:44 AM Samuel Holland <samuel@sholland.org> wrote:
>> On 8/3/20 9:02 AM, Arnd Bergmann wrote:
>>> On Mon, Aug 3, 2020 at 5:42 AM Samuel Holland <samuel@sholland.org> wrote:
>>>> All of the command structures are packed, due to the "#pragma pack(1)" earlier
>>>> in the file. So alignment is not an issue. This dma_addr_t member _is_ the
>>>> explicit padding to make sizeof(TW_Command) -
>>>> sizeof(TW_Command.byte8_offset.{io,param}.sgl) equal TW_COMMAND_SIZE * 4. And
>>>> indeed the structure is expected to be a different size depending on
>>>> sizeof(dma_addr_t).
>>>
>>> Ah, so only the first few members are accessed by hardware and the
>>> last union is only accessed by the OS then? In that case I suppose it is
>>> all fine, but I would also suggest removing the "#pragma packed"
>>> to get somewhat more efficient access on systems that have  problems
>>> with misaligned accesses.
>>
>> I don't know what part the hardware accesses; everything I know about the
>> hardware comes from reading the driver.
> 
> I see now from your explanation below that this is a hardware-defined
> structure. I was confused by how it can be either 32 or 64 bits wide but
> found the
> 
> tw_initconnect->features |= sizeof(dma_addr_t) > 4 ? 1 : 0;
> 
> line now that tells the hardware about which format is used.
> 
>> The problem with removing the "#pragma pack(1)" is that the structure is
>> inherently misaligned: byte8_offset.io.sgl starts at offset 12, but it may begin
>> with a __le64.
> 
> I think a fairly clean way to handle this would be to remove the pragma
> and instead define a local type like
> 
> #if IS_ENABLED(CONFIG_ARCH_DMA_ADDR_T_64BIT)
> typedef  __le64 twa_address_t __packed;
> #else
> typedef __le32 twa_addr_t;
> #endif

I would be happy to implement this... but __packed only works on enums, structs,
and unions[1]:

In file included from drivers/scsi/3w-9xxx.c:100:
drivers/scsi/3w-9xxx.h:474:1: warning: 'packed' attribute ignored [-Wattributes]
  474 | typedef __le64 twa_addr_t __packed;
      | ^~~~~~~

[1]:
https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html#index-packed-type-attribute

> The problem with marking the entire structure as packed, rather than
> just individual members is that you end up with very inefficient bytewise
> access on some architectures (especially those without cache-coherent
> DMA or hardware unaligned access in the CPU), so I would recommend
> avoiding that in portable driver code.

I agree, but I think this is a separate issue from what this patch is fixing. I
would prefer to save this change for a separate patch.

Regards,
Samuel