question on mmc_queue_bounce_pre()/post() in block.c

question on mmc_queue_bounce_pre()/post() in block.c

[J Freyensee]

I have a question concerning the following block of code in 
~/drivers/mmc/card/block.c:

		mmc_queue_bounce_pre(mq);

		/*
		 * Before issuing a user req, host driver should
		 * wait for the BKOPS is done or just use HPI to
		 * interrupt it.
		 */
		if (mmc_wait_for_bkops(card))
			goto cmd_err;
		mmc_wait_for_req(card->host, &brq.mrq);
		mmc_queue_bounce_post(mq);

What is the purpose of mmc_queue_bounce_pre() and 
mmc_queue_bounce_post()?  At first when I looked at the code, I thought 
it was implementing some type of sw cache- mmc_queue_bounce_pre(mq) 
would be stored in a buffer, available for reads even before the data 
gets eventually written to the SSD from the mmc_wait_for_req() call. 
But the way the code is written, a read from the buffer won't happen 
until AFTER the mmc_wait_for_req(), which kind-of defeats the purpose of 
cache.  This is independent of the work Per Forlin and co. are doing to 
make mmc_wait_for_req() non-blocking. So with the way that the code is 
written today, I am not really sure what benefit 
mmc_queue_bounce_pre()/*_post() is providing.

I'm still ramping up in this area of the linux kernel, like I don't yet 
understand the purpose of a scatterlist.  But I'm looking to contribute 
to this area of Linux in some regard.

Thanks for the help,
Jay