Re: [PATCH 2/2] iommu: mtk_iommu: Add support for MT6795 Helio X10 M4Us

[Yong Wu <yong.wu@mediatek.com>]

On Tue, 2022-05-17 at 11:26 +0200, AngeloGioacchino Del Regno wrote:
> Il 17/05/22 11:08, Yong Wu ha scritto:
> > On Fri, 2022-05-13 at 17:14 +0200, AngeloGioacchino Del Regno
> > wrote:
> > > Add support for the M4Us found in the MT6795 Helio X10 SoC.
> > > 
> > > Signed-off-by: AngeloGioacchino Del Regno <
> > > angelogioacchino.delregno@collabora.com>
> > > ---
> > >   drivers/iommu/mtk_iommu.c | 20 +++++++++++++++++++-
> > >   1 file changed, 19 insertions(+), 1 deletion(-)

[...]

> > > +static const struct mtk_iommu_plat_data mt6795_data = {
> > > +	.m4u_plat     = M4U_MT6795,
> > > +	.flags	      = HAS_4GB_MODE | HAS_BCLK | RESET_AXI |
> > > +			HAS_LEGACY_IVRP_PADDR | MTK_IOMMU_TYPE_MM,
> > > +	.inv_sel_reg  = REG_MMU_INV_SEL_GEN1,
> > > +	.banks_num    = 1,
> > > +	.banks_enable = {true},
> > > +	.iova_region  = single_domain,
> > > +	.iova_region_nr = ARRAY_SIZE(single_domain),
> > > +	.larbid_remap = {{0}, {1}, {2}, {3}, {4}}, /* Linear mapping.
> > > */
> > > +};
> > 
> > This is nearly same with mt8173_data. mt8173 has one more larb than
> > mt6795, its larbid_remap is also ok for mt6795.
> > 
> 
> I think that we should be explicit about the larbid_remap property,
> since mt6795 has one less larb, we should explicitly say that like
> I did there... that's only for human readability I admit ... but,
> still, I wouldn't want to see people thinking that MT6795 has 6 LARBs
> because they've read that larbid_remap having 6 entries.

In the linear mapping case, It does help. Strictly larbid_remap is two-
dimensional array now, It doesn't indicate how many larbs this SoC
has. If someone would like to know this, he could read the mtxxx-larb-
port.h. We also could document the larb numbers in the binding
for readability.

Anyway, It is not a big problem. A new structure is ok for me. I just
complain there are too many structures, specially in the internal
branch which contains many non-upstream SoCs, and there may be several
IOMMU HWs in one SoC. thus I'd like to group it personally.

> 
> > thus it looks we could use mt8173 as the backward compatible.
> >      compatible = "mediatek,mt6795-m4u",
> >                   "mediatek,mt8173-m4u";
> > 
> > After this, the only thing is about "mediatek,mt6795-infracfg". we
> > have
> > to try again with mediatek,mt6795-infracfg after mediatek,mt8173-
> > infracfg fail. I think we should allow the backward case in 4GB
> > mode
> > judgment if we have.
> > 
> > What's your opinion? or some other suggestion?
> > Thanks.
> 
> I know, I may have a plan for that, but I wanted to have a good
> reason to
> propose such a thing, as if it's just about two SoCs needing that,
> there
> would be no good reason to get things done differently.
> 
> ...so, in order to provide a good cleanup, we have two possible roads
> to
> follow here: either we add a generic "mediatek,infracfg" compatible
> to the
> infra node (but I don't like that), or we can do it like it was
> previously
> done in mtk-pm-domains.c (I prefer that approach):
> 
> iommu: iommu@somewhere {
> 	... something ...
> 	mediatek,infracfg = <&infracfg>;

We like this too. But this was not suggested from Rob long before.

https://lore.kernel.org/linux-mediatek/20200715205120.GA778876@bogus/

> };
> 
> infracfg = syscon_regmap_lookup_by_compatible(node,
> "mediatek,infracfg");
> if (IS_ERR(infracfg)) {
> 	/* try with the older way */
> 	switch (...) {
> 	case .... p = "mediatek,mt2712-infracfg";
> 	... blah blah ...
> 	}
> 	/* legacy also failed, ouch! */
> 	if (IS_ERR(infracfg))
> 		return PTR_ERR(infracfg);
> }
> 
> ret = regmap_read ... etc etc etc
> 
> Cheers,
> Angelo