Re: [PATCH 1/2] iommu/io-pgtable: Add MTK 4GB mode in Short-descriptor

From: Robin Murphy
Date: Wed Mar 02 2016 - 07:31:53 EST


Hi Yong,

On 23/02/16 23:02, Yong Wu wrote:
Mediatek extend bit9 in the lvl1 and lvl2 pgtable descriptor of the
Short-descriptor as the 4GB mode in which the dram size will be
over 4GB.

We add a special quirk for this MTK-4GB mode, And in the standard
spec, Bit9 in the lvl1 is "IMPLEMENTATION DEFINED", while it's AP[2]
in the lvl2, therefore if this quirk is enabled, NO_PERMS is also
expected.

Would you be able to explain exactly what this "4GB mode" actually is? I've been trying to make sense of it from the original M4U patches and the patch for the I2C driver, but it has me completely baffled.

Is it simply used as an extra physical address bit (although surely that would make it "8GB mode"?), or does it do something crazier like toggling an interconnect remap that shifts the output addresses up by 1GB to be relative to the base of DRAM, like a dma_pfn_offset?

I guess from the look of these patches that it doesn't change anything between the masters and the M4U, so input addresses are still 32 bits and we don't need extended tables, right? Furthermore, what about the TTBRs? Does the level 1 table still have to be below 4GB?

Signed-off-by: Yong Wu <yong.wu@xxxxxxxxxxxx>
---
In arm_v7s_init_pte, We add bit9 if the 4GB mode is enabled no matter
the current pa is over 4GB or not.

Either way I can't comprehend how it could be fine to just enable unconditionally without doing _something_ with the actual addresses.

drivers/iommu/io-pgtable-arm-v7s.c | 14 +++++++++++++-
drivers/iommu/io-pgtable.h | 6 ++++++
2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/io-pgtable-arm-v7s.c b/drivers/iommu/io-pgtable-arm-v7s.c
index 9fcceb1..bf6a6f0 100644
--- a/drivers/iommu/io-pgtable-arm-v7s.c
+++ b/drivers/iommu/io-pgtable-arm-v7s.c
@@ -121,6 +121,8 @@
#define ARM_V7S_TEX_MASK 0x7
#define ARM_V7S_ATTR_TEX(val) (((val) & ARM_V7S_TEX_MASK) << ARM_V7S_TEX_SHIFT)

+#define ARM_V7S_ATTR_MTK_4GB BIT(9) /* MTK extend it for 4GB mode */
+
/* *well, except for TEX on level 2 large pages, of course :( */
#define ARM_V7S_CONT_PAGE_TEX_SHIFT 6
#define ARM_V7S_CONT_PAGE_TEX_MASK (ARM_V7S_TEX_MASK << ARM_V7S_CONT_PAGE_TEX_SHIFT)
@@ -364,6 +366,9 @@ static int arm_v7s_init_pte(struct arm_v7s_io_pgtable *data,
if (lvl == 1 && (cfg->quirks & IO_PGTABLE_QUIRK_ARM_NS))
pte |= ARM_V7S_ATTR_NS_SECTION;

+ if (cfg->quirks & IO_PGTABLE_QUIRK_MTK_4GB_EXT)
+ pte |= ARM_V7S_ATTR_MTK_4GB;
+
if (num_entries > 1)
pte = arm_v7s_pte_to_cont(pte, lvl);

@@ -625,9 +630,16 @@ static struct io_pgtable *arm_v7s_alloc_pgtable(struct io_pgtable_cfg *cfg,

if (cfg->quirks & ~(IO_PGTABLE_QUIRK_ARM_NS |
IO_PGTABLE_QUIRK_NO_PERMS |
- IO_PGTABLE_QUIRK_TLBI_ON_MAP))
+ IO_PGTABLE_QUIRK_TLBI_ON_MAP |
+ IO_PGTABLE_QUIRK_MTK_4GB_EXT))
return NULL;

+ /* If MTK_4GB_EXT is enabled, the NO_PERMS is also expected. */
+ if (cfg->quirks & IO_PGTABLE_QUIRK_MTK_4GB_EXT) {
+ if (!(cfg->quirks & IO_PGTABLE_QUIRK_NO_PERMS))

Ha, I wasn't expecting to see Will's argument against generic quirk-checking be vindicated quite so soon :)

Anyway, no need for braces and nested ifs:

if (cfg->quirks & IO_PGTABLE_QUIRK_MTK_4GB_EXT &&
!(cfg->quirks & IO_PGTABLE_QUIRK_NO_PERMS))

+ return NULL;
+ }
+
data = kmalloc(sizeof(*data), GFP_KERNEL);
if (!data)
return NULL;
diff --git a/drivers/iommu/io-pgtable.h b/drivers/iommu/io-pgtable.h
index d4f5027..a84a60a 100644
--- a/drivers/iommu/io-pgtable.h
+++ b/drivers/iommu/io-pgtable.h
@@ -60,10 +60,16 @@ struct io_pgtable_cfg {
* IO_PGTABLE_QUIRK_TLBI_ON_MAP: If the format forbids caching invalid
* (unmapped) entries but the hardware might do so anyway, perform
* TLB maintenance when mapping as well as when unmapping.
+ *
+ * IO_PGTABLE_QUIRK_MTK_4GB_EXT: Mediatek extend bit9 in the lvl1 and
+ * lvl2 descriptor of the Short-descriptor as the 4GB mode.
+ * Note that: Bit9 in the lvl1 is "IMPLEMENTATION DEFINED", while
+ * it is AP[2] in the lvl2.

Unfortunately that comment doesn't really explain anything - I'd be happy to suggest a more helpful wording, If only I understood what it actually did.

Robin.

*/
#define IO_PGTABLE_QUIRK_ARM_NS BIT(0)
#define IO_PGTABLE_QUIRK_NO_PERMS BIT(1)
#define IO_PGTABLE_QUIRK_TLBI_ON_MAP BIT(2)
+ #define IO_PGTABLE_QUIRK_MTK_4GB_EXT BIT(3)
unsigned long quirks;
unsigned long pgsize_bitmap;
unsigned int ias;