Messages in this thread | | | Date | Tue, 16 Dec 2003 19:12:23 -0500 (EST) | From | "Richard B. Johnson" <> | Subject | Re: PCI Express support for 2.4 kernel |
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On Wed, 17 Dec 2003, Vladimir Kondratiev wrote:
> Arjan van de Ven wrote: > > >>>+ /* dummy read to flush PCI write */ > >>>+ readb(addr); > >>> > >>> > >>This is going to choke some hardware, I guarantee. > >> > >>You always want to make sure your flush is of the same size at the > >>write. Reading a byte from an address that the hardware defines as > >>"32-bit writes only" can get ugly real quick ;-) > >> > >> > > > >also reading back addr might not be the best choice in case some > >registers have side effects on reading, it's probably better to read > >back an address that is known to be ok to read (like the vendor ID > >field) > > > > > > > Good idea!
You should not abitrarily flush all the writes with a read after each write. The PCI/Bus is a FIFO, stuff will get to the hardware in the order written. If you "flush" every write, you will seriously hurt the performance. I don't like that MACRO/Function, whatever it is with the switch shown previously. All that code just for a "@(*^&$(*^(" write will screw up performance. I've looked at the result of some the the MACRO expansions, in particular the get_user, put_user, and copy_to/from macros where there is a switch. It is not pre-processed out.
Cheers, Dick Johnson Penguin : Linux version 2.4.22 on an i686 machine (797.90 BogoMips). Note 96.31% of all statistics are fiction.
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