Re: [PATCH 9/9] drm/ttm: Sanity check mapping attributes on powerpc in ttm_io_prot()

On 04.09.2014 16:47, Benjamin Herrenschmidt wrote:

On all current cache coherent powerpc processors, it is not legit to map system memory non-cachable. This will cause aliases with the linear mapping which can be fatal.

The TTM should generally avoid it after Jerome placement patches but let's add a sanity check anyway to catch any possible remaining issue.

Signed-off-by: Benjamin Herrenschmidt benh@palm4.ozlabs.ibm.com

[...]

@@ -498,6 +501,20 @@ pgprot_t ttm_io_prot(uint32_t caching_flags, pgprot_t tmp) tmp = pgprot_noncached(tmp);

#endif +#if defined(__powerpc__) && !defined(CONFIG_NOT_COHERENT_CACHE)

• /*

• * Using a non-cachable mapping of system memory on
  

• * cache coherent powerpc's can be fatal, let's make
  

• * sure this doesn't happen and warn if it does. The
  

• * only exception is powermac with AGP which has to
  

• * take the risk.
  

• */
  

• if (!machine_is(powermac) &&

•    ((caching_flags & TTM_PL_FLAG_SYSTEM) ||
  

•     (caching_flags & TTM_PL_FLAG_TT))) {
  

• pr_err_once("TTM: Attempt to use a non-cached"
  

• 	    " mapping on RAM unsupported !\n");
  

• return tmp;
  

NAK, this breaks AGP on PowerMacs.