question on PPC performance

[VanBaren, Gerald (AGRE)]

...because you are NOT accessing SDRAM when you are accessing SDRAM.  You are accessing the processor's internal cache, which is running at 200MHz.  In fact, you probably are not even accessing internal cache -- IIRC, the 82xx has a write posting queue so, when you do a write, it gets queued but does not wait for the write to complete.  When you do a read of the same location, the bus interface goes gets the value out of the write queue, so you are only reading and writing a hidden internal register in the 82xx, not SDRAM and probably not even the internal cache.

Turn off the data cache and run your test again, you will be astounded at how slow it is.

When you are accessing a port, you need a bus transaction.  As your benchmark showed, it takes more time to access a built-in port via the 82xx internal bus.  Since your bus is clocked at 66MHz and you are measuring 15nS, it appears that accessing the built-in 82xx ports is running at the bus speed.

Your benchmark is very, very simplistic and you are getting correspondingly simplistic measurements.  I don't see any calculation or compensation in your benchmark for the overhead of the loop.  As pointed out above, it is not measuring what you think it is measuring.

gvb


> -----Original Message-----
> From: owner-linuxppc-embedded at lists.linuxppc.org
> [mailto:owner-linuxppc-embedded at lists.linuxppc.org]On Behalf Of John
> Zhou
> Sent: Friday, January 30, 2004 6:32 AM
> To: 'linuxppc-embedded'
> Subject: RE: question on PPC performance
>
>
>
> my questions is:
>
> why is it 15 ns when accessing other devices such as port
> A/B/C/D, immr, etc, except accessing SDRAM, but about 5 ns
> when accessing SDRAM?
>
> Thanks in advance!
> John
>
> -----Original Message-----
> From: owner-linuxppc-embedded at lists.linuxppc.org
> [mailto:owner-linuxppc-embedded at lists.linuxppc.org]On Behalf Of John
> Zhou
> Sent: Friday, January 30, 2004 6:22 PM
> To: 'linuxppc-embedded'
> Subject: question on PPC performance
>
>
>
> Dear all,
>
> I have an question on PPC performance:
>
> I have linux running based on mpc8250 with 200/166/66 clock
> configuration. But, I found that every CPU instruction used
> about 5 ns when accessing SDRAM, otherwise, every CPU
> instruction used about 15 ns when accessing other devices
> such as port A/B/C/D, immr, etc, except accessing SDRAM.
>
> Thanks any help!
>
> John
> =================================================
> The function I used is:
> void performance_test(void)
> {
> 					unsigned long i, d1, d2;
> 					static unsigned long kkk;
> 					unsigned short time;
> 					volatile unsigned long*
> portC = ( volatile unsigned long*)0xF0010D50;
>
> 					volatile unsigned long
> * tmp = &kkk;
> 					*(volatile unsigned
> char*)0xF0010D80 &= ~0xB0;
> 					*(volatile unsigned
> short*)0xF0010D92 = 0x0002;
> 					*(volatile unsigned
> char*)0xF0010D80 |= 0x10;
>
>  					*(volatile unsigned
> long*)0xF0010D44 &= ~0x00000002;
> 					d1 = *(volatile
> unsigned long*)0xF0010D50;
> 					d1 &= ~0x00000003;
> 					d2 = d1 | 0x00000002;
> 					d1 |= 0x00000001;
>
> 					*(volatile unsigned
> short*)0xF0010D9E = 0;
> 					for (i=0; i<1000; i++)
> 					{
>
> 						*portC = d1;
> 						*portC = d2;
> 					}
> 					time = *(volatile
> unsigned short*)0xF0010D9E;
> 					printk ("#test2: 2
> access loop 1000 times use %dns\r\n", time*15);
>
> 					*(volatile unsigned
> short*)0xF0010D9E = 0;
> 					for (i=0; i<1000; i++)
> 					{
>
> 						*tmp = d1;
> 						*tmp = d2;
> 					}
> 					time = *(volatile
> unsigned short*)0xF0010D9E;
> 					printk ("#test3: 2
> access loop 1000 times use %dns\r\n", time*15);
> }
>
>


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