[PATCH 1/2] dm: core: add support for fallback drivers

[Caleb Connolly]

On 10/04/2024 19:27, Heinrich Schuchardt wrote:
> 
> 
> Am 10. April 2024 19:06:57 MESZ schrieb Caleb Connolly <caleb.connolly at linaro.org>:
>> Introduce support for a uclass to provide a fallback/stub driver which
>> can be used when no device is found for a given node. This might be
>> useful for handling non-essential clock controllers like the RPMh on
>> Qualcomm platforms, or during early bringup to get UART output before a
>> real clock driver has been created.
>>
>> Signed-off-by: Caleb Connolly <caleb.connolly at linaro.org>
>> ---
>> drivers/core/Kconfig  | 10 ++++++++++
>> drivers/core/uclass.c | 24 +++++++++++++++++++++++-
>> include/dm/uclass.h   |  3 +++
>> 3 files changed, 36 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/core/Kconfig b/drivers/core/Kconfig
>> index 1081d61fcf01..09075b9b7a15 100644
>> --- a/drivers/core/Kconfig
>> +++ b/drivers/core/Kconfig
>> @@ -466,5 +466,15 @@ config BOUNCE_BUFFER
>>
>> 	  A second possible use of bounce buffers is their ability to
>> 	  provide aligned buffers for DMA operations.
>>
>> +menuconfig FALLBACK_DRIVERS
> 
> Wouldn't it be preferable to mark individual drivers as fallback drivers in their declaration?
> 
> This would allow alternative fallback drivers and would not require any definitions at uclass level.

I don't see an obvious usecase for having multiple stub drivers for a 
given uclass. Maybe as a result of that, I have no idea how we'd go 
about choosing which stub to use if multiple were available.

I don't think it would be particularly hard to switch over if at some 
point in the future we have such a need, but I don't see a benefit to 
this added complexity now.
> 
> Just by building a fallback driver you would enable the fallback behavior for its uclass.

I could drop the CONFIG_FALLBACK_DRIVERS option, it's just there to make 
it possible to opt in/out of all stub drivers at once, but maybe this 
isn't very useful.

Kind Regards,
> 
> Best regards
> 
> Heinrich
> 
>> +	bool "Enable per-uclass fallback drivers"
>> +	depends on DM
>> +	help
>> +	  If a driver requests a resource (like a clock) from a node which
>> +	  isn't bound to a driver, the driver model will look for a fallback
>> +	  driver to "stub" the resource. These stubs usually do nothing and
>> +	  are therefore only suitable in instances where the resource is not
>> +	  required.
>> +
>> endmenu
>> diff --git a/drivers/core/uclass.c b/drivers/core/uclass.c
>> index e46d5717aa62..91d3a48d77b8 100644
>> --- a/drivers/core/uclass.c
>> +++ b/drivers/core/uclass.c
>> @@ -378,8 +378,26 @@ int uclass_find_device_by_of_offset(enum uclass_id id, int node,
>>
>> 	return -ENODEV;
>> }
>>
>> +static int uclass_bind_fallback(struct uclass *uc, ofnode node, struct udevice **devp)
>> +{
>> +	struct driver *drv;
>> +
>> +	log(LOGC_DM, LOGL_ERR, "   - binding fallback '%s' driver '%s'\n",
>> +	    uc->uc_drv->name, uc->uc_drv->fallback_drv_name);
>> +
>> +	drv = lists_driver_lookup_name(uc->uc_drv->fallback_drv_name);
>> +	if (!drv) {
>> +		log(LOGC_DM, LOGL_DEBUG, "   - Can't find stub driver '%s' for uclass '%s'\n",
>> +		    uc->uc_drv->fallback_drv_name, uc->uc_drv->name);
>> +		return -ENOENT;
>> +	}
>> +
>> +	return device_bind_with_driver_data(gd->dm_root, drv,
>> +					  ofnode_get_name(node), 0, node, devp);
>> +}
>> +
>> int uclass_find_device_by_ofnode(enum uclass_id id, ofnode node,
>> 				 struct udevice **devp)
>> {
>> 	struct uclass *uc;
>> @@ -401,9 +419,13 @@ int uclass_find_device_by_ofnode(enum uclass_id id, ofnode node,
>> 			*devp = dev;
>> 			goto done;
>> 		}
>> 	}
>> -	ret = -ENODEV;
>> +
>> +	if (CONFIG_IS_ENABLED(FALLBACK_DRIVERS) && uc->uc_drv->fallback_drv_name)
>> +		ret = uclass_bind_fallback(uc, node, devp);
>> +	else
>> +		ret = -ENODEV;
>>
>> done:
>> 	log(LOGC_DM, LOGL_DEBUG, "   - result for %s: %s (ret=%d)\n",
>> 	    ofnode_get_name(node), *devp ? (*devp)->name : "(none)", ret);
>> diff --git a/include/dm/uclass.h b/include/dm/uclass.h
>> index 456eef7f2f31..b99e36485bc5 100644
>> --- a/include/dm/uclass.h
>> +++ b/include/dm/uclass.h
>> @@ -67,8 +67,10 @@ struct udevice;
>>   * @child_pre_probe: Called before a child in this uclass is probed
>>   * @child_post_probe: Called after a child in this uclass is probed
>>   * @init: Called to set up the uclass
>>   * @destroy: Called to destroy the uclass
>> + * @stub_drv_name: Name of a stub driver to use for devices that are not
>> + * supported by any other driver.
>>   * @priv_auto: If non-zero this is the size of the private data
>>   * to be allocated in the uclass's ->priv pointer. If zero, then the uclass
>>   * driver is responsible for allocating any data required.
>>   * @per_device_auto: Each device can hold private data owned
>> @@ -98,8 +100,9 @@ struct uclass_driver {
>> 	int (*child_pre_probe)(struct udevice *dev);
>> 	int (*child_post_probe)(struct udevice *dev);
>> 	int (*init)(struct uclass *class);
>> 	int (*destroy)(struct uclass *class);
>> +	const char *fallback_drv_name;
>> 	int priv_auto;
>> 	int per_device_auto;
>> 	int per_device_plat_auto;
>> 	int per_child_auto;
>>
> 

-- 
// Caleb (they/them)