Re: question regarding cacheline size

[linux-os (Dick Johnson)]

On Thu, 7 Sep 2006, Tejun Heo wrote:

> Hello,
>
> This is for PCMCIA (cardbus) version of Silicon Image 3124 SerialATA
> controller.  When cacheline size is configured, the controller uses Read
> Multiple commands.
>
> • Bit [07:00]: Cache Line Size (R/W). This bit field is used to specify
> the system cacheline size in terms of 32-bit words. The SiI3124, when
> initiating a read transaction, will issue the Read Multiple PCI command
> if empty space in its FIFO is greater than the value programmed in this
> register.
>
> As the BIOS doesn't run after hotplugging cardbus card, the cache line
> isn't configured and the controller ends up having 0 cache line size and
> always using Read command.  When that happens, write performance drops
> hard - the throughput is < 2Mbytes/s.
>
> http://thread.gmane.org/gmane.linux.ide/12908/focus=12908
>
> So, sata_sil24 driver has to program CLS if it's not already set, but
> I'm not sure which number to punch in.  FWIW, sil3124 doesn't seem to
> put restrictions on the values which can be used for CLS.  There are
> several candidates...
>
> * L1_CACHE_BYTES / 4 : this is used by init routine in yenta_socket.c.
> It seems to be a sane default but I'm not sure whether L1 cache line
> size always coincides with the size as seen from PCI bus.
>
> * pci_cache_line_size in drivers/pci/pci.c : this is used for
> pci_generic_prep_mwi() and can be overridden by arch specific code.
> this seems more appropriate but is not exported.
>
> For all involved commands - memory read line, memory read multiple and
> memory write and invalidate - a value larger than actual cacheline size
> doesn't hurt but a smaller value may.
>
> I'm thinking of implementing a query function for pci_cache_line_size,
> say, int pci_cacheline_size(struct pci_dev *pdev), and use it in the
> device init routine.  Does this sound sane?
>
> Thanks.
>
> --
> tejun

The cache line size specifies the system cache-line size in dword
increments. For most, (ix86) this would be 8, i.e., eight 32-bit
words or 32 bytes. This is from page 376, PCI System Architecture,
ISBN 0-201-30974-2. It also says that a device may limit the number
of cache cycles if an unsupported value is written there. In that
case, the device will act as if the value 0 was written (no write-and-
invalidate transactions), basically poor performance.

The L1 cache size shouldn't have anything to do with this, BTW.


Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.24 on an i686 machine (5592.66 BogoMips).
New book: http://www.AbominableFirebug.com/
_


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