On 2018/10/30 1:59, Will Deacon wrote:
> On Sat, Oct 20, 2018 at 03:36:54PM +0800, Zhen Lei wrote:
>> The standard GITS_TRANSLATER register in ITS is only 4 bytes, but
>> Hisilicon expands the next 4 bytes to carry some IMPDEF information. That
>> means, total 8 bytes data will be written to MSIAddress each time.
>>
>> MSIAddr: |----4bytes----|----4bytes----|
>> | MSIData | IMPDEF |
>>
>> There is no problem for ITS, because the next 4 bytes space is reserved
>> in ITS. But it will overwrite the 4 bytes memory following "sync_count".
>> It's very fortunately that the previous and the next neighbour of the
>> "sync_count" are both aligned by 8 bytes, so no problem is met now.
>>
>> It's good to explicitly add a workaround:
>> 1. Add gcc __attribute__((aligned(8))) to make sure that "sync_count" is
>> always aligned by 8 bytes.
>> 2. Add a "int" struct member to make sure the 4 bytes padding is always
>> exist.
>>
>> There is no functional change.
>>
>> Signed-off-by: Zhen Lei <thunder.leiz...@huawei.com>
>> ---
>> drivers/iommu/arm-smmu-v3.c | 15 ++++++++++++++-
>> 1 file changed, 14 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
>> index 5059d09..624fdd0 100644
>> --- a/drivers/iommu/arm-smmu-v3.c
>> +++ b/drivers/iommu/arm-smmu-v3.c
>> @@ -586,7 +586,20 @@ struct arm_smmu_device {
>>
>> struct arm_smmu_strtab_cfg strtab_cfg;
>>
>> - u32 sync_count;
>> + /*
>> + * The alignment and padding is required by Hi16xx of Hisilicon.
>> + * Because the ITS hardware on Hi16xx will truncate the MSIAddress(Here
>> + * it's the address of "sync_count") to 8 bytes boundary first, then
>> + * write 32 bits MSIdata at offset 0, and 32 bits IMPDEF data at offset
>> + * 4. Without this workaround, the adjacent member maybe overwritten.
>> + *
>> + * |---4bytes---|---4bytes---|
>> + * MSIAddress & (~0x7): MSIdata | IMPDEF data|
>> + */
>> + struct {
>> + u32 sync_count;
>> + int padding;
>> + } __attribute__((aligned(8)));
>
> I thought the conclusion after reviewing your original patch was to maintain
> the union and drop the alignment directive? e.g.
>
> union {
> u32 sync_count;
> u64 padding; /* Hi16xx writes an extra 32 bits of goodness
> */
> };
OK, I will sent v3.
>
> Will
>
> .
>
--
Thanks!
BestRegards
