[linux-dvb] Re: Slow i2c problem

[Michael Hunold <hunold@convergence.de>]

Hello Robert,

> I'd say the i2c routines are just poorly written, or rather, the hardware
> was not fully understood. Look at this comment in the source:
Yes, yes, bash me already... 8-)

> /* reason: while fiddling around with the i2c-routines, I noticed
>    that after writing out an i2c-command, one may not read out the
>    status immediately after that. you *must* wait some time, before
>    even the busy-flag gets set */
>
> I, too, noticed that the PSR does not immediately reflect the I2C busy
> status, but the fix is *FAR* simpler than that: Simply read the PSR *once*
> and *junk* the result. Reading it a *second* time always resulted in the
> proper I2C busy status for me.
The "old" av7110 driver (in DVB) was forked from an early version of my 
saa7146. From a time, where I did not fully understand the hardware and 
had quite poor kernel programming knowledge.

> But take a look what the i2c_busy_rise_and_fall() function does instead: It
> reads the PSR *once* and if the "I2C busy" bit is not set (which it never
> is upon the first read!) it does:
It fully admit that this stuff is utterly crap. It was improved during 
my work on the video4linux-2 saa7146 driver and is now in dvb-kernel. 
It's still far from being optimal, but it works.

For the analog video cards, i2c is only used for non-critical stuff, so 
even the old code was good-enough most of the time.

> Yes, rewrite the SAA7146A I2C functions from scratch :) Alternatively, try
> putting just an extra read of the PSR into the i2c_busy_rise_and_fall()
> function before the loops, that might work wonders already.
Please have a look at dvb-kernel, too. If anyone fixes up the i2c 
routines, I'll apply the patches.

The only problem is, that the code must work with all old analog tv 
cards I have, too, which use various i2c helper chipsets. (The 
Multimedia eXtension Board, the dpc7146 Demonstration board, the Hexium 
Orion and Gemini frame grabbers, ...)

I once added irq based i2c transmission, but this only works for non-DVB 
cards. If you do have TS grabbing and use irq based i2c for reading the 
frontend status for example, the additionaly irqs screw up the TS 
capture irqs, resulting in debi oops messages and TS lockup.

CU
Michael.



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