[PATCH 0/4] More SD / GPS noise reduction

Andy Green andy at openmoko.com
Thu Jul 31 19:45:26 CEST 2008

Hash: SHA1

Somebody in the thread at some point said:
| The buzzing that I hear during phone calls has noticeably changed
| character and gotten quieter since the last round of sd fixes went in.
| It seems to happen more often before, and used to sound like continuous
| GSM radio buzz.  Now it's just a periodic "click click click" 10-100
| times a second.  Don't know how its changed things for people on the
| other end of the call.  Also, I don't make many phone calls, so I could
| have just gotten lucky recently.
| I have a question about reducing the sd card clock.  sd access is done
| without DMA, right?  Doesn't that mean that slow factor 8 increases the
| cpu cost of reading data from SD by 8x?

Yes, we pull the data from Glamo without DMA.  No, reducing SD Card
clock doesn't impact CPU side at all.

We get the Glamo to pull the bulk data into its RAM autonomously and
interrupt us when it's done.  In the interrupt service code we spam the
data back from Glamo RAM to CPU RAM as fast as we can, then mark the
transaction as completed in the MCI layer.  That part doesn't care how
long it took to get the data into Glamo RAM.

The change just means it takes longer before we get the interrupt
signalling we can pull the data from Glamo RAM.  In the meanwhile, we
are idle or running another process, etc, not spinning waiting.

- -Andy
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