[PATCH 0/5] drm/solomon: Add SSD130x OLED displays SPI support

[PATCH 0/5] drm/solomon: Add SSD130x OLED displays SPI support

[Javier Martinez Canillas]

Hello,

This series adds a ssd130x-spi driver that provides a 4-wire SPI transport
support for SSD130x OLED controllers that can be accessed through a SPI.

The driver is quite similar to existing ssd130x-i2c driver that is used by
I2C controllers, but there is a difference in the protocol used by SSD130x
depending on the transport used. The details are in patch #4 description.

Patch #1 just makes the current ssd130x-i2c compatible strings in the DT
binding to be deprecated, and add new ones that don't have an -fb suffix.

Patch #2 extends the DT binding with the compatible string and properties
needed to support the ssd130x-spi devices.

Patch #3 adds the new compatible strings to the OF device ID table in the
ssd130x-i2c DRM driver.

Patch #4 moves the device info for the different SSD130x variants from
the ssd130x-i2c transport driver to the ssd130x core driver.

Finally patch #5 adds the ssd130x-spi DRM driver for the OLED controllers
that come with a 4-wire SPI interface, instead of an I2C interface.

Best regards,
Javier


Javier Martinez Canillas (5):
  dt-bindings: display: ssd1307fb: Deprecate fbdev compatible strings
  dt-bindings: display: ssd1307fb: Extend schema for SPI controllers
  drm/solomon: Add ssd130x-i2c compatible strings without an -fb suffix
  drm/solomon: Move device info from ssd130x-i2c to the core driver
  drm/solomon: Add SSD130x OLED displays SPI support

 .../bindings/display/solomon,ssd1307fb.yaml   | 117 ++++++++---
 drivers/gpu/drm/solomon/Kconfig               |   9 +
 drivers/gpu/drm/solomon/Makefile              |   1 +
 drivers/gpu/drm/solomon/ssd130x-i2c.c         |  60 +++---
 drivers/gpu/drm/solomon/ssd130x-spi.c         | 184 ++++++++++++++++++
 drivers/gpu/drm/solomon/ssd130x.c             |  60 +++++-
 drivers/gpu/drm/solomon/ssd130x.h             |  13 ++
 7 files changed, 376 insertions(+), 68 deletions(-)
 create mode 100644 drivers/gpu/drm/solomon/ssd130x-spi.c

-- 
2.35.1

[PATCH 1/5] dt-bindings: display: ssd1307fb: Deprecate fbdev compatible strings

[Javier Martinez Canillas]

The current compatible strings for SSD130x I2C controllers contain an -fb
suffix, this seems to indicate that are for a fbdev driver. But the DT is
supposed to describe the hardware and not Linux implementation details.

Let's deprecate those compatible strings and add a new enum that contains
compatible strings that don't have a -fb suffix. These will be matched by
the ssd130x-i2c DRM driver.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
---

 .../bindings/display/solomon,ssd1307fb.yaml   | 36 ++++++++++++-------
 1 file changed, 24 insertions(+), 12 deletions(-)

diff --git a/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml b/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml
index ade61d502edd..46207f2c12b8 100644
--- a/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml
+++ b/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml
@@ -12,12 +12,24 @@ maintainers:
 
 properties:
   compatible:
-    enum:
-      - sinowealth,sh1106-i2c
-      - solomon,ssd1305fb-i2c
-      - solomon,ssd1306fb-i2c
-      - solomon,ssd1307fb-i2c
-      - solomon,ssd1309fb-i2c
+    oneOf:
+      # Deprecated compatible strings
+      - items:
+          - enum:
+              - solomon,ssd1305fb-i2c
+              - solomon,ssd1306fb-i2c
+              - solomon,ssd1307fb-i2c
+              - solomon,ssd1309fb-i2c
+        deprecated: true
+
+      # SSD130x I2C controllers
+      - items:
+          - enum:
+              - sinowealth,sh1106-i2c
+              - solomon,ssd1305-i2c
+              - solomon,ssd1306-i2c
+              - solomon,ssd1307-i2c
+              - solomon,ssd1309-i2c
 
   reg:
     maxItems: 1
@@ -148,7 +160,7 @@ allOf:
       properties:
         compatible:
           contains:
-            const: solomon,ssd1305fb-i2c
+            const: solomon,ssd1305-i2c
     then:
       properties:
         solomon,dclk-div:
@@ -160,7 +172,7 @@ allOf:
       properties:
         compatible:
           contains:
-            const: solomon,ssd1306fb-i2c
+            const: solomon,ssd1306-i2c
     then:
       properties:
         solomon,dclk-div:
@@ -172,7 +184,7 @@ allOf:
       properties:
         compatible:
           contains:
-            const: solomon,ssd1307fb-i2c
+            const: solomon,ssd1307-i2c
     then:
       properties:
         solomon,dclk-div:
@@ -186,7 +198,7 @@ allOf:
       properties:
         compatible:
           contains:
-            const: solomon,ssd1309fb-i2c
+            const: solomon,ssd1309-i2c
     then:
       properties:
         solomon,dclk-div:
@@ -203,14 +215,14 @@ examples:
             #size-cells = <0>;
 
             ssd1307: oled@3c {
-                    compatible = "solomon,ssd1307fb-i2c";
+                    compatible = "solomon,ssd1307-i2c";
                     reg = <0x3c>;
                     pwms = <&pwm 4 3000>;
                     reset-gpios = <&gpio2 7>;
             };
 
             ssd1306: oled@3d {
-                    compatible = "solomon,ssd1306fb-i2c";
+                    compatible = "solomon,ssd1306-i2c";
                     reg = <0x3c>;
                     pwms = <&pwm 4 3000>;
                     reset-gpios = <&gpio2 7>;
-- 
2.35.1

[PATCH 2/5] dt-bindings: display: ssd1307fb: Extend schema for SPI controllers

[Javier Martinez Canillas]

The Solomon SSD130x OLED displays can either have an I2C or SPI interface,
add to the schema the compatible strings, properties and examples for SPI.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
---

 .../bindings/display/solomon,ssd1307fb.yaml   | 89 +++++++++++++++----
 1 file changed, 71 insertions(+), 18 deletions(-)

diff --git a/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml b/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml
index 46207f2c12b8..05e7975296a7 100644
--- a/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml
+++ b/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml
@@ -31,6 +31,15 @@ properties:
               - solomon,ssd1307-i2c
               - solomon,ssd1309-i2c
 
+      # SSD130x SPI controllers
+      - items:
+          - enum:
+              - sinowealth,sh1106-spi
+              - solomon,ssd1305-spi
+              - solomon,ssd1306-spi
+              - solomon,ssd1307-spi
+              - solomon,ssd1309-spi
+
   reg:
     maxItems: 1
 
@@ -40,9 +49,14 @@ properties:
   reset-gpios:
     maxItems: 1
 
+  dc-gpios:
+    maxItems: 1
+
   vbat-supply:
     description: The supply for VBAT
 
+  spi-max-frequency: true
+
   solomon,height:
     $ref: /schemas/types.yaml#/definitions/uint32
     default: 16
@@ -148,19 +162,10 @@ allOf:
       properties:
         compatible:
           contains:
-            const: sinowealth,sh1106-i2c
-    then:
-      properties:
-        solomon,dclk-div:
-          default: 1
-        solomon,dclk-frq:
-          default: 5
-
-  - if:
-      properties:
-        compatible:
-          contains:
-            const: solomon,ssd1305-i2c
+            enum:
+              - sinowealth,sh1106-i2c
+              - solomon,ssd1305-i2c
+              - solomon,ssd1305-spi
     then:
       properties:
         solomon,dclk-div:
@@ -172,7 +177,9 @@ allOf:
       properties:
         compatible:
           contains:
-            const: solomon,ssd1306-i2c
+            enum:
+              - solomon,ssd1306-i2c
+              - solomon,ssd1306-spi
     then:
       properties:
         solomon,dclk-div:
@@ -184,7 +191,9 @@ allOf:
       properties:
         compatible:
           contains:
-            const: solomon,ssd1307-i2c
+            enum:
+              - solomon,ssd1307-i2c
+              - solomon,ssd1307-spi
     then:
       properties:
         solomon,dclk-div:
@@ -198,7 +207,9 @@ allOf:
       properties:
         compatible:
           contains:
-            const: solomon,ssd1309-i2c
+            enum:
+              - solomon,ssd1309-i2c
+              - solomon,ssd1309-spi
     then:
       properties:
         solomon,dclk-div:
@@ -206,6 +217,21 @@ allOf:
         solomon,dclk-frq:
           default: 10
 
+  - if:
+      properties:
+        compatible:
+          contains:
+            enum:
+              - sinowealth,sh1106-spi
+              - solomon,ssd1305-spi
+              - solomon,ssd1306-spi
+              - solomon,ssd1307-spi
+              - solomon,ssd1309-spi
+    then:
+      required:
+        - spi-max-frequency
+        - dc-gpios
+
 additionalProperties: false
 
 examples:
@@ -214,14 +240,14 @@ examples:
             #address-cells = <1>;
             #size-cells = <0>;
 
-            ssd1307: oled@3c {
+            ssd1307_i2c: oled@3c {
                     compatible = "solomon,ssd1307-i2c";
                     reg = <0x3c>;
                     pwms = <&pwm 4 3000>;
                     reset-gpios = <&gpio2 7>;
             };
 
-            ssd1306: oled@3d {
+            ssd1306_i2c: oled@3d {
                     compatible = "solomon,ssd1306-i2c";
                     reg = <0x3c>;
                     pwms = <&pwm 4 3000>;
@@ -232,3 +258,30 @@ examples:
                     solomon,lookup-table = /bits/ 8 <0x3f 0x3f 0x3f 0x3f>;
             };
     };
+  - |
+    spi {
+            #address-cells = <1>;
+            #size-cells = <0>;
+
+            ssd1307_spi: oled@0 {
+                    compatible = "solomon,ssd1307-spi";
+                    reg = <0x0>;
+                    pwms = <&pwm 4 3000>;
+                    reset-gpios = <&gpio2 7>;
+                    dc-gpios = <&gpio2 8>;
+                    spi-max-frequency = <10000000>;
+            };
+
+            ssd1306_spi: oled@1 {
+                    compatible = "solomon,ssd1306-spi";
+                    reg = <0x1>;
+                    pwms = <&pwm 4 3000>;
+                    reset-gpios = <&gpio2 7>;
+                    dc-gpios = <&gpio2 8>;
+                    spi-max-frequency = <10000000>;
+                    solomon,com-lrremap;
+                    solomon,com-invdir;
+                    solomon,com-offset = <32>;
+                    solomon,lookup-table = /bits/ 8 <0x3f 0x3f 0x3f 0x3f>;
+            };
+    };
-- 
2.35.1

Re: [PATCH 1/5] dt-bindings: display: ssd1307fb: Deprecate fbdev compatible strings

[Rob Herring]

On Thu, Apr 07, 2022 at 10:02:00PM +0200, Javier Martinez Canillas wrote:
> The current compatible strings for SSD130x I2C controllers contain an -fb
> suffix, this seems to indicate that are for a fbdev driver. But the DT is
> supposed to describe the hardware and not Linux implementation details.

True, but compatible is just an identifier. There's no reason to 
deprecate unless the binding as a whole needs to be redone.

I imagine you also want 2 compatibles for 2 drivers. That's saying you 
should change your firmware to switch drivers. The fact that we have 2 
drivers for the same h/w is a kernel problem. Don't bring DT into it.

> Let's deprecate those compatible strings and add a new enum that contains
> compatible strings that don't have a -fb suffix. These will be matched by
> the ssd130x-i2c DRM driver.
> 
> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
> ---
> 
>  .../bindings/display/solomon,ssd1307fb.yaml   | 36 ++++++++++++-------
>  1 file changed, 24 insertions(+), 12 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml b/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml
> index ade61d502edd..46207f2c12b8 100644
> --- a/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml
> +++ b/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml
> @@ -12,12 +12,24 @@ maintainers:
>  
>  properties:
>    compatible:
> -    enum:
> -      - sinowealth,sh1106-i2c
> -      - solomon,ssd1305fb-i2c
> -      - solomon,ssd1306fb-i2c
> -      - solomon,ssd1307fb-i2c
> -      - solomon,ssd1309fb-i2c
> +    oneOf:
> +      # Deprecated compatible strings
> +      - items:
> +          - enum:
> +              - solomon,ssd1305fb-i2c
> +              - solomon,ssd1306fb-i2c
> +              - solomon,ssd1307fb-i2c
> +              - solomon,ssd1309fb-i2c
> +        deprecated: true
> +
> +      # SSD130x I2C controllers
> +      - items:
> +          - enum:
> +              - sinowealth,sh1106-i2c
> +              - solomon,ssd1305-i2c
> +              - solomon,ssd1306-i2c
> +              - solomon,ssd1307-i2c
> +              - solomon,ssd1309-i2c

There's also no reason to put the bus interface into the compatible as 
the same compatible will work on different buses. But since you want to 
add SPI, just using the 'i2c' one will confuse people. For that reason 
you could add 'solomon,ssd1305', etc. for both SPI support and I2C DRM. 
(You should also support the 'fb-i2c' variant in DRM IMO, but doubtful 
that I'll review that.)

Rob

Re: [PATCH 1/5] dt-bindings: display: ssd1307fb: Deprecate fbdev compatible strings

[Javier Martinez Canillas]

Hello Rob,

On 4/8/22 20:22, Rob Herring wrote:
> On Thu, Apr 07, 2022 at 10:02:00PM +0200, Javier Martinez Canillas wrote:
>> The current compatible strings for SSD130x I2C controllers contain an -fb
>> suffix, this seems to indicate that are for a fbdev driver. But the DT is
>> supposed to describe the hardware and not Linux implementation details.
> 
> True, but compatible is just an identifier. There's no reason to 
> deprecate unless the binding as a whole needs to be redone.
> 
> I imagine you also want 2 compatibles for 2 drivers. That's saying you 
> should change your firmware to switch drivers. The fact that we have 2 
> drivers for the same h/w is a kernel problem. Don't bring DT into it.
>

No, that's not what I meant. In fact, we currently have two drivers that
match against the same set of compatible strings. These drivers are:

* drivers/video/fbdev/ssd1307fb.c
* drivers/gpu/drm/solomon/ssd130x-i2c.c

what I don't personally like about the current compatible strings is that
the *driver* name was encoded on those, rather than the IC names. So for
instance there's a "solomon,ssd1307fb-i2c" (notice the fb suffix) instead
of just "solomon,ssd1307-i2c" or "solomon,ssd1307".

When I ported the fbdev driver to DRM, I considered using different values
for the compatible strings but decided to just use the same for backward
compatibility.

But now I want to add compatible strings for OLED controllers that use a
SPI interface instead, and I don't really want to add a compatible string
"solomon,ssd1307fb-spi" but just without the "fb".

I want the SPI compatible strings to be consistent with the I2C ones though,
hence the deprecation so new DTS could just use the ones without a "fb".

Now, if you say that I can't do add new ones for I2C, then I will just add
"solomon,ssd1307fb-spi" and similar even though I don't like the "fb" there.

And just to make clear, the DRM driver will continue matching against both
compatible strings, but the fbdev will only match the old "fb" ones.

[snip]

>> +
>> +      # SSD130x I2C controllers
>> +      - items:
>> +          - enum:
>> +              - sinowealth,sh1106-i2c
>> +              - solomon,ssd1305-i2c
>> +              - solomon,ssd1306-i2c
>> +              - solomon,ssd1307-i2c
>> +              - solomon,ssd1309-i2c
> 
> There's also no reason to put the bus interface into the compatible as 
> the same compatible will work on different buses. But since you want to 
> add SPI, just using the 'i2c' one will confuse people. For that reason 
> you could add 'solomon,ssd1305', etc. for both SPI support and I2C DRM.

That's not really true. There's a reason to add per bus compatible strings
at least in Linux. And is that there's no information about the bus types
in module aliases that are reported to user-space for module auto-loading.

For example, 

$ cat /sys/devices/platform/soc/fe804000.i2c/i2c-1/1-003c/modalias 
of:NoledT(null)Csolomon,ssd1306fb-i2c

$ cat /sys/devices/platform/soc/fe804000.i2c/i2c-1/1-003c/uevent 
DRIVER=ssd130x-i2c
OF_NAME=oled
OF_FULLNAME=/soc/i2c@7e804000/oled@3c
OF_COMPATIBLE_0=solomon,ssd1306fb-i2c
OF_COMPATIBLE_N=1
MODALIAS=of:NoledT(null)Csolomon,ssd1306fb-i2c

and

$ modinfo ssd130x-i2c | grep alias
alias:          of:N*T*Csolomon,ssd1309fb-i2cC*
alias:          of:N*T*Csolomon,ssd1309fb-i2c
alias:          of:N*T*Csolomon,ssd1307fb-i2cC*
alias:          of:N*T*Csolomon,ssd1307fb-i2c
alias:          of:N*T*Csolomon,ssd1306fb-i2cC*
alias:          of:N*T*Csolomon,ssd1306fb-i2c
alias:          of:N*T*Csolomon,ssd1305fb-i2cC*
alias:          of:N*T*Csolomon,ssd1305fb-i2c
alias:          of:N*T*Csinowealth,sh1106-i2cC*
alias:          of:N*T*Csinowealth,sh1106-i2c

this module will match against any MODALIAS uevent that has one of the
listed compatible strings in "C" and any node name in "N". But also for
any type "T".

And even if the module alias was more restrictive and say only matched
against 'of:N*Ti2cCsolomon,ssd1307fb-i2c', the type information is not
filled by the bus drivers.

So, if we just had a "solomon,ssd1307" compatible string, then a device
registered through OF could lead to the wrong kernel module to be loaded.

In other words, it's true that having a single compatible strings for all
bus drivers will work for device -> driver matching but may not work for
module auto-loading.

> (You should also support the 'fb-i2c' variant in DRM IMO, but doubtful 
> that I'll review that.)
>

As mentioned above, it does even after adding support for the new strings,
for backward compatibility.
 
> Rob
> 

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat

Re: [PATCH 1/5] dt-bindings: display: ssd1307fb: Deprecate fbdev compatible strings

[Javier Martinez Canillas]

On 4/8/22 21:19, Javier Martinez Canillas wrote:

[snip]

>>
>> There's also no reason to put the bus interface into the compatible as 
>> the same compatible will work on different buses. But since you want to 
>> add SPI, just using the 'i2c' one will confuse people. For that reason 
>> you could add 'solomon,ssd1305', etc. for both SPI support and I2C DRM.
> 
> That's not really true. There's a reason to add per bus compatible strings
> at least in Linux. And is that there's no information about the bus types
> in module aliases that are reported to user-space for module auto-loading.
>

Forgot to mention that in this particular case it will work but just because
the SPI subsystem always report a module alias of the form "spi:device" even
for devices that are registered through OF.

So having a single "solomon,ssd1306" would work because for I2C the module
alias will be "of:NoledT(null)Csolomon,ssd1306" and for SPI it will be
"spi:ssd1306".

But if ever the SPI subsystem is fixed to report proper OF module aliases
things will break. And since the DT bindings is an ABI, it's safer to have
"-i2c" and "-spi" compatible strings variants.

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat

Re: [PATCH 1/5] dt-bindings: display: ssd1307fb: Deprecate fbdev compatible strings

[Geert Uytterhoeven]

Hi Javier,

On Thu, Apr 7, 2022 at 10:03 PM Javier Martinez Canillas
<javierm@redhat.com> wrote:
> The current compatible strings for SSD130x I2C controllers contain an -fb
> suffix, this seems to indicate that are for a fbdev driver. But the DT is
> supposed to describe the hardware and not Linux implementation details.
>
> Let's deprecate those compatible strings and add a new enum that contains
> compatible strings that don't have a -fb suffix. These will be matched by
> the ssd130x-i2c DRM driver.
>
> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>

> --- a/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml
> +++ b/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml
> @@ -12,12 +12,24 @@ maintainers:
>
>  properties:
>    compatible:
> -    enum:
> -      - sinowealth,sh1106-i2c
> -      - solomon,ssd1305fb-i2c
> -      - solomon,ssd1306fb-i2c
> -      - solomon,ssd1307fb-i2c
> -      - solomon,ssd1309fb-i2c
> +    oneOf:
> +      # Deprecated compatible strings
> +      - items:
> +          - enum:
> +              - solomon,ssd1305fb-i2c
> +              - solomon,ssd1306fb-i2c
> +              - solomon,ssd1307fb-i2c
> +              - solomon,ssd1309fb-i2c

Please drop the "-i2c" suffixes, too.
We already have plenty of IIO sensors and audio codecs using the
same compatible value for spi and i2c, cfr.
'git grep compatible -- "*-[si][p2][ic].c"'

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Re: [PATCH 1/5] dt-bindings: display: ssd1307fb: Deprecate fbdev compatible strings

[Javier Martinez Canillas]

Hello Geert,

On 4/11/22 15:47, Geert Uytterhoeven wrote:
> Hi Javier,
> 
> On Thu, Apr 7, 2022 at 10:03 PM Javier Martinez Canillas
> <javierm@redhat.com> wrote:
>> The current compatible strings for SSD130x I2C controllers contain an -fb
>> suffix, this seems to indicate that are for a fbdev driver. But the DT is
>> supposed to describe the hardware and not Linux implementation details.
>>
>> Let's deprecate those compatible strings and add a new enum that contains
>> compatible strings that don't have a -fb suffix. These will be matched by
>> the ssd130x-i2c DRM driver.
>>
>> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
> 
>> --- a/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml
>> +++ b/Documentation/devicetree/bindings/display/solomon,ssd1307fb.yaml
>> @@ -12,12 +12,24 @@ maintainers:
>>
>>  properties:
>>    compatible:
>> -    enum:
>> -      - sinowealth,sh1106-i2c
>> -      - solomon,ssd1305fb-i2c
>> -      - solomon,ssd1306fb-i2c
>> -      - solomon,ssd1307fb-i2c
>> -      - solomon,ssd1309fb-i2c
>> +    oneOf:
>> +      # Deprecated compatible strings
>> +      - items:
>> +          - enum:
>> +              - solomon,ssd1305fb-i2c
>> +              - solomon,ssd1306fb-i2c
>> +              - solomon,ssd1307fb-i2c
>> +              - solomon,ssd1309fb-i2c
> 
> Please drop the "-i2c" suffixes, too.
> We already have plenty of IIO sensors and audio codecs using the
> same compatible value for spi and i2c, cfr.
> 'git grep compatible -- "*-[si][p2][ic].c"'
>

Yes, I know but was worried about the potential issues that mentioned in a
previous email in this thread. But after the discussion we had over IRC, I
think that is safe to assume that the SPI subsystem won't change how the
modaliases are reported, so there won't be conflict between I2C and SPI.

And if that is ever changed, there's a plan to add the bus type to the data
reported by the modalias uevent so user-space could figure out what to load.

So I'll go ahead with Rob and yours suggestion, and just deprecate the old
ones and drop both the "fb" and "-i2c" part of the compatible strings, to
use the same compatible strings for both the I2C and SPI drivers.

After all, that's the correct way to describe the hardware and not encode
any Linux implementation details in the DT binding.

-- 
Best regards,

Javier Martinez Canillas
Linux Engineering
Red Hat