Re: IO FLASH and caches

[Edgar Grimberg <edgar.grimberg@zylin.com>]

Hi Nick,

Thanks for the feedback.

On Fri, Oct 16, 2009 at 1:04 PM, Nick Garnett <nickg@ecoscentric.com> wrote:
> Edgar Grimberg <edgar.grimberg@zylin.com> writes:
>
>> Hi,
>>
>> The attached patch handles the case where the CPU cannot disable
>> caches and needs to provide an uncached address instead. The fix was
>> tested with a NIOS2 CPU.
>
> I don't think this patch is correct. In general flash should be read
> through the cache. Flash devices are slow and using the cache improves
> performance. It is the responsibility of the flash driver to ensure
> that stale data is evicted from the caches by calling cache control
> macros. In general the drivers already do this.
>
> This patch forces uncached access for any architecture that defines
> CYGARC_UNCACHED_ADDRESS(). Many platforms (e.g. MIPS, PPC, ARM9)
> provide both cached and uncached views of memory. These architectures
> also provide mechanisms for flushing and invalidating the caches.

The only place where the uncached address is used is inside the area guarded by:

HAL_FLASH_CACHES_OFF(d_cache, i_cache);
...
HAL_FLASH_CACHES_ON(d_cache, i_cache);

So the patch is not supposed to modify the intended access type.

>
> As far as I am aware, the NIOS2 has cache control instructions, so I
> am not sure what the actual reason for this patch is.

 NIOS2 cannot just turn the caches off. The only way to say that a
memory range should not be accessed via caches is to set the MSB bit
of the address to 1.

> In any case, I don't think that other architectures should be forced
> to operate at lower performance just to accommodate the shortcomings
> of one particular architecture.

Agree. I'm not sure that's the case, though.

Regards,
Edgar

>
> --
> Nick Garnett                                       eCos Kernel Architect
> eCosCentric Limited    http://www.eCosCentric.com       The eCos experts
> Barnwell House, Barnwell Drive, Cambridge, UK.      Tel: +44 1223 245571
> Registered in England and Wales:                         Reg No: 4422071
>
>



-- 
Edgar Grimberg
System Developer
Zylin AS
ZY1000 JTAG Debugger http://www.zylin.com/zy1000.html
Phone: (+47) 51 63 25 00