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

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Date:	Thu, 26 Jun 2014 20:03:20 +0100
From:	Sudeep Holla <sudeep.holla@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Sudeep Holla <sudeep.holla@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <robh@...nel.org>,
	"linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"x86@...nel.org" <x86@...nel.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"linux390@...ibm.com" <linux390@...ibm.com>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/9] drivers: base: support cpu cache information interface
 to userspace via sysfs



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

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