Re: [PATCH 2/9] drivers: base: support cpu cache information interface to userspace via sysfs

Sudeep Holla
2014-06-26T19:03:20Z

On 26/06/14 19:50, Russell King - ARM Linux wrote:
> On Thu, Jun 26, 2014 at 07:41:32PM +0100, Sudeep Holla wrote:
>> Hi,
>>
>> On 25/06/14 23:23, Russell King - ARM Linux wrote:
>>> On Wed, Jun 25, 2014 at 06:30:37PM +0100, Sudeep Holla wrote:
>>>> + coherency_line_size: the minimum amount of data that gets transferred
>>>
>>> So, what value to do envision this taking for a CPU where the cache
>>> line size is 32 bytes, but each cache line has two dirty bits which
>>> allow it to only evict either the upper or lower 16 bytes depending
>>> on which are dirty?
>>>
>>
>> IIUC most of existing implementations of cacheinfo on various architectures
>> are representing the cache line size as coherency_line_size, in which case I
>> need fix the definition in this file.
>
> As an example, here's an extract from the SA110 TRM:
>
> StrongARM contains a 16KByte writeback data cache. The DC has 512 lines
> of 32 bytes (8 words), arranged as a 32 way set associative cache, and
> uses the virtual addresses generated by the processor. A line also
> contains the physical address the block was fetched from and two dirty
> bits. There is a dirty bit associated with both the first and second
> half of the block. When a store hits in the cache the dirty bit
> associated with it is set. When a block is evicted from the cache the
> dirty bits are used to decide if all, half, or none of the block will
> be written back to memory using the physical address stored with the
> block. The DC is always reloaded a line at a time (8 words).
>

Thanks for the information. It's interesting that line is referred as block
when referring to 2 dirty bits. I am not sure if this can be mapped to
physical_line_partition = 2. Thoughts ?

>> BTW will there be any architectural way of finding such configuration ?
>
> Not that I know of.
>
That's bad :)

Regards,
Sudeep