Re: [PATCH 2/2] iommu/omap: Add support for configuring dsp iommus on DRA7xx

[Suman Anna <s-anna-l0cyMroinI0@public.gmane.org>]

Hi Tony,

On 07/23/2015 02:24 AM, Tony Lindgren wrote:
> * Suman Anna <s-anna-l0cyMroinI0@public.gmane.org> [150722 09:25]:
>> On 07/22/2015 12:26 AM, Tony Lindgren wrote:
>>>
>>> I don't like using syscon for tinkering directly with SoC registers.
>>
>> This is not a SoC-level register, but a register within a sub-module of
>> the DSP processor sub-system. The DSP_SYSTEM sub-module in general is
>> described in Section 5.3.3 of the TRM [1], and it implements different
>> functionalities like the PRCM handshaking, wakeup logic and DSP
>> subsystem top-level configuration. It is a module present within the DSP
>> processor sub-system, so can only be accessed when the sub-system is
>> clocked and the appropriate reset is released.
> 
> OK so if it's specific to the DSP driver along the lines of sysc and
> syss registers.

There will be those registers too within the MMU config register space,
even for DRA7xx MMUs. This is different, think of it like a register in
the Control module except that it is present within the DSP sub-system
instead of at the SoC level.

> 
> Typically we handle these registers by mapping them to the PM runtime
> functions for the interconnect so we can reset and idle the hardware
> modules even if there is no driver, see for example
> omap54xx_mmu_dsp_hwmod.

I haven't yet submitted the DRA7xx hwmods, but they will look almost
exactly like the OMAP5 ones. The reset and idle on these are in general
not effective at boot time since these are also controlled by a
hard-reset line, so that's left to the drivers to deal with it through
the omap_device_deassert_hardreset API.

>  
>>> We should use some Linux generic framework for configuring these
>>> bits to avoid nasty dependencies between various hardware modules
>>> on the SoC.
>>>
>>> What does DSP_SYS_MMU_CONFIG register do? It seems it's probably
>>> a regulator or a gate clock? If so, it should be set up as a
>>> regulator or a clock and then the omap-iommu driver can just
>>> use regulator or clcok framework to request the resource.
>>
>> No, its neither. It is a control bit that dictates whether the
>> processor/EDMA addresses go through the respective MMU or not. The
>> register currently has 4 bits (bit 0 in each nibble), one each for
>> enabling each MMU and requesting an MMU abort on each. The MMU
>> integration and enablement notes are detailed in Section 5.3.6 of the
>> TRM [1], and the DSP_SYS_MMU_CONFIG register layout is in Table 5-28
>> (Page 1641).
> 
> OK yeah seems like it should be handled by the DSP driver during
> probe after doing pm_runtime_get. Then the driver can configure
> things like IOMMU and load firmware. Or am I missing something here?

The DSP (remoteproc) driver uses the generic IOMMU API, and all the
IOMMU enabling sequence is transparent to the DSP driver. So, any
configuration/enabling sequence specific to the IOMMU has to be handled
within the respective IOMMU platform driver that gets integrated into
the IOMMU API.

regards
Suman

> 
>  
>> [1] http://www.ti.com/lit/ug/spruhz6/spruhz6.pdf