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SubjectRe: NatSemi Geode improvement
Date
In message "Re: NatSemi Geode improvement"
on 02/10/21, Alan Cox <alan@lxorguk.ukuu.org.uk> writes:
> > + printk(KERN_INFO "Enable Memory access reorder on Cyrix/NSC processor.\n");
> > + local_irq_save(flags);
> > + ccr3 = getCx86(CX86_CCR3);
> > + setCx86(CX86_CCR3, (ccr3 & 0x0f) | 0x10); /* enable MAPEN */
> > + /* Load/Store Serialize to mem access disable (=reorder it) */
> > + setCx86(CX86_PCR0, getCx86(CX86_PCR0) & ~0x80);
> > +#ifdef CONFIG_NOHIGHMEM
> > + /* set load/store serialize from 1GB to 4GB */
> > + ccr3 |= 0xe0;
> > +#endif
> > + setCx86(CX86_CCR3, ccr3);
>
> I dont think this is safe. You now need store fences on bus mastering
> DMA. You should be able to reuse the IDT winchip code for that - I set
> the winchip up for weak store ordering too, and its a big win (I also
> saw about 30% on block copies)

Winchip C6 MCR is like to intel MTRR and Cyrix 6x86MX ARR.
Geode has NO similar registers and only has serialize flags to 1-2GB,2-3GB,3-4GB
which are the CCR3's MSB bits. Geode is always serialize the 640KB-1MB area.
The read/write reordering and the read bypassing are handled by geode MMU.

It means that mmio must map to over 1GB area or disable this feature.

for example, on my geode machine,

$ cat /proc/iomem
00000000-0009fbff : System RAM
0009fc00-0009ffff : reserved
000a0000-000bffff : Video RAM area
000c0000-000c7fff : Video ROM
000f0000-000fffff : System ROM
00100000-09d7ffff : System RAM
00100000-001e7ce2 : Kernel code
001e7ce3-0022a857 : Kernel data
10000000-10000fff : Ricoh Co Ltd RL5c475
10000000-10000fff : i82365
40010000-40010fff : Cyrix Corporation 5520 [Cognac]
60000000-60000fff : card services
e0000000-e0000fff : Compaq Computer Corporation ZFMicro Chipset USB
e0000000-e0000fff : usb-ohci
ffff0000-ffffffff : reserved
(this sample is on linux-2.4.19-pre10-ac2)

with this, this patch may safe in this condition.

anyway, this reorder setting can be done by 'set6x86' tool,
so user can decide in user space whether use the mem re-ordering or not.


Hiroshi Miura --- http://www.da-cha.org/
NTTDATA Corp. Marketing & Business Strategy Planning Dept. --- miurahr@nttdata.co.jp
Key fingerprint = 9117 9407 5684 FBF1 4063 15B4 401D D077 04AB 8617
-- My hacking life is happy as the day is long



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